c:\program files\Common Files can't be seen in windows xp

Are you logged in as an administrator? These files are only viewable as an admin. Most likely you will need to run as admin.

what happens if you run start -> run - > %CommonProgramFiles%

If it can be seen in safe mode, then it is probably some startup program that runs in normal mode but not safe mode that is inhibiting your seeing of that folder.  In Windows 98/ME/XP/Vista/Windows 7, you can choose which programs are run at startup by using the Microsoft Configuration utility:  click on the Start button, choose Run, type in MSCONFIG (doesn't need to be all capitals), click OK.  Choose the Startup tab, deselect programs to run at startup by removing the check mark.  In Windows 9x/ME, you may need to disable just about everything except Explorer, ScanRegistry and SysTray.  In Windows XP/Vista/Windows 7, just click on the button for Disable All. Reboot and see if the problem goes away.  If it does then re-enable the startup programs one (or a few) at a time, reboot, and when the problem occurs again, there is your culprit.  If you have a great number of apps to narrow down on in trying to track down the culprit, do it instead by dividing up the loading programs into successively smaller halves:  disable half of them; if the problem doesn't go away, then the offending app is within the other half, so divide that group into half; or if the problem does go away, then the disabled group contains the offender, so again narrow down by dividing in half.  

In Windows XP/Vista/Windows 7: If the problem continues to come up with all startup programs disabled, then it is probably a driver or service.  You can click on the Services tab in MSCONFIG, click on the button for Hide All Microsoft Services, leaving only third-party services displayed, then try turning them off in the same way you did for startup programs.  Finally, if third-party services are eliminated from being the cause of the problem, you could try narrowing down on Microsoft services in the same way.

Disabled startup but no go

executed %CommonProgramFiles% - Error Windows Can't 'c:\Program'.

Yes I am logged in as Administrator

The folder is there , because If I try to create a folder called common files, XP tells me I can't use that name because folder exists.

At the command prompt try

cd %CommonProgramFiles%

cd "C:\Program Files\Common Files"

Another way to get the variable...

Start>run>cmd

set c<enter>

Should see "CommonProgramFiles" listed....

it worked