cpu usage 100 per cent

You havent given us any information at all ?

Download a program called Hijack this

Post your results

And whats using all the cpu ?

with this info, the answer to your question can be : buy a new PC !
Depending on how long it is you had a new install, you must realize that a fresh install cures all these problems.
also - i see many sp3 problems reported here; so if it started after the upgrade : uninstall upgrades 1 by 1 and test
or use a system restore to previous date(s)

Thank you for you responses. I attach the Process Explorer report and the Hijackthis log file. The computer is an Intel  D915GeV  with a Pentium 4 3.4 Ghz with 2Gb RAM and 2 x 500Gb hard drives. The Video card is an Nvidia GeForce 7300GT and the sound comes from an external Creative Labs 24bit sound box.The computer is now running almost constantly at 100% CPU usage even after i have disabled most of the optional startup programs and services. The computer is virtually useless in its present state and just getting this information to you guys is quite a mission. I hope you can make something of it. Best Regards, Athol
Process-Explorer-image-07-11-200.png
hijackthis-20081107A.log

I'd suggest go into safe mode first (By Pressing F8 during Windows startup), if your problem goes away during safe mode,you are likely to be able to fix the problem by uninstalling the troubled application.

Then boot normally, find out the process which is using the CUP, uninstall the related program.

Also do a full system virus scan, some virus do that too.
Also, I have seen AV scanners use 100% CPU time.

Hope that helps.

these are reported as bad or may be fixed: but make a backup first !
C:\Program Files\Conceptworld\RecentX\RecentX.exe
O1 - Hosts: 66.98.136.25 auto.search.msn.es
O2 - BHO: Media Holding Enterprises, LLC - {0D39A900-0F3A-4C29-A254-3E65244FDC34} - (no file)
Must be fixed!
Unnecessary (deactivated) entry that can be fixed. ContextTool-1.dll, ContextTool-2.dll, ContextTool-3.dll - Adware bundled with music playing software from playmp3z.biz, http://www.playmp3z.biz/ 
O2 - BHO: WormRadar.com IESiteBlocker.NavFilter - {3CA2F312-6F6E-4B53-A66E-4E65E497C8C0} - C:\Program Files\AVG\AVG8\avgssie.dll (file missing)
Neutral
Unnecessary (deactivated) entry that can be fixed. LinkScannerIE.dll - LinkScanner, http://linkscanner.explabs.com/linkscann er/default.asp
O2 - BHO: (no name) - {4E7BD74F-2B8D-469E-85AA-FD60BB9AAE22} - (no file)

Unnecessary (deactivated) entry that can be fixed. uetoolbar.dll, UE_TOO~1.DLL - UltraEdit, http://ultraedit.com/index.php?name=Cont ent&pa=showpage&pid=10 toolbar
O2 - BHO: (no name) - {7E853D72-626A-48EC-A868-BA8D5E23E045} - (no file)
Safe
Unnecessary (deactivated) entry that can be fixed. This entry was classified from our visitors as good.
O3 - Toolbar: (no name) - {4E7BD74F-2B8D-469E-85AA-FD60BB9AAE22} - (no file)

Unnecessary (deactivated) entry that can be fixed. uetoolbar.dll, UE_TOO~1.DLL - UltraEdit, http://ultraedit.com/index.php?name=Cont ent&pa=showpage&pid=10 toolbar
O3 - Toolbar: QT TabBar - {d2bf470e-ed1c-487f-a333-2bd8835eb6ce} - mscoree.dll (file missing)
Very safe
Unnecessary (deactivated) entry that can be fixed. mscoree.dll - QT TabBar, http://members.at.infoseek.co.jp/Quizo/f reeware/indexEn.html
O3 - Toolbar: QT Tab Standard Buttons - {D2BF470E-ED1C-487F-A666-2BD8835EB6CE} - mscoree.dll (file missing)
Very safe
Unnecessary (deactivated) entry that can be fixed. mscoree.dll - QT TabBar, http://quizo.at.infoseek.co.jp/freeware/ indexEn.html
O3 - Toolbar: xplorer2 Toolbar - {db35fda8-77e3-4784-92c2-ee7345e91af4} - C:\Program Files\xplorer2\tbxplo.dll
Unknown application

O4 - HKUS\S-1-5-18\..\RunServices: [Windows System Security] sys32.pif (User 'SYSTEM')
Must be fixed! Added by the RBOT-AOL WORM!
O4 - HKUS\.DEFAULT\..\RunServices: [Windows System Security] sys32.pif (User 'Default user')
Must be fixed! Added by the RBOT-AOL WORM!

O8 - Extra context menu item: WebResearch: Save Link Address As... - res://C:\PROGRA~1\WEBRES~3\wrshell.dll/#110
The entry WebResearch has been identified as nasty.    O8 - Extra context menu item: WebResearch: Save Page Area (Frame) - res://C:\PROGRA~1\WEBRES~3\wrshell.dll/#102
The entry WebResearch has been identified as nasty.    O8 - Extra context menu item: WebResearch: Save Page Area (Frame) As... - res://C:\PROGRA~1\WEBRES~3\wrshell.dll/#106
The entry WebResearch has been identified as nasty.    O8 - Extra context menu item: WebResearch: Save Picture - res://C:\PROGRA~1\WEBRES~3\wrshell.dll/#101
The entry WebResearch has been identified as nasty.    O8 - Extra context menu item: WebResearch: Save Picture As... - res://C:\PROGRA~1\WEBRES~3\wrshell.dll/#108
The entry WebResearch has been identified as nasty.    O8 - Extra context menu item: WebResearch: Save Selected Targets As... - res://C:\PROGRA~1\WEBRES~3\wrshell.dll/#111
The entry WebResearch has been identified as nasty.    O8 - Extra context menu item: WebResearch: Save Selection - res://C:\PROGRA~1\WEBRES~3\wrshell.dll/#104
The entry WebResearch has been identified as nasty.    O8 - Extra context menu item: WebResearch: Save Selection As... - res://C:\PROGRA~1\WEBRES~3\wrshell.dll/#109
The entry WebResearch has been identified as nasty.    O8 - Extra context menu item: WebResearch: Save Target - res://C:\PROGRA~1\WEBRES~3\wrshell.dll/#103
The entry WebResearch has been identified as nasty.    O8 - Extra context menu item: WebResearch: Save Target As... - res://C:\PROGRA~1\WEBRES~3\wrshell.dll/#107
The entry WebResearch has been identified as nasty.
  O15 - Trusted Zone: *.download.com
Safe If you did not add these pages to your trusted pages, they should be fixed.

i recommend running ALL these :
     adaware :      http://www.lavasoftusa.com/
     Spybot :        http://www.download.com/3000-8022-10122137.html
http://housecall.trendmicro.com/                                                               online scan for trojans

Thank you Nobus for your analysis of my Hijackthis file and your suggestions. I have done these checks and of course found some viruses and problem areas that certainly needed urgent attention. However, the improvement to the CPU usage was small and it still frequently spikes at 100%. From further investigation on the Internet and enquiries from the local gurus I  was led to investigate the CPU temperature which is running at 88 degreec Celcius - way over its limit of about 62 degrees. I have all the fans etc that can be installed and am now in the process of a complete hardware cleanup and applying thermal paste again on the heatsink. I am told by the local tech. that the Intel Prescot 3.4 Ghz always had a cooling problem and is unable to offer an improved heatsink. Any suggestion for a very effective heatsink? Any other way to keep this rig cool. Thank you once again.

zalman -  they have the best cooling :
http://www.zalman.co.kr/ENG/product/CategorySecond_Pic.asp?categoryname=Coolers&categorySecond=CAS1
>>  way over its limit of about 62 degrees   <<  this will cause the slowness, since intel cpu's will go to lower speed (stepdown feature)

or do you like this :  http://www.tomshardware.com/reviews/strip-fans,1203.html

After installing a Gigabyte G-Power GH-PDU21-MF heat sink & cooler unit, it reduced the CPU temperature to a very acceptable 37 degrees thus increasing the life of the capacitors etc. However, for such a drastic solution the improvement in the CPU usage was very modest. After following GARETH22'S advice of installing the printers, re-booting & reinstalling them, the CPU is now idling at 2% and very occasionally under severe load it spikes above 50%. The computer is now stable & performing well. Thank you for your simple, effective and pertinent solution.