"Loading your personal settings" Then logging off - same in safe mode.

Could be the profile is corrupt.  Can you log in as a different user?

The registry may be corrupt.  Take a look at this question I just answered a few minutes ago:

https://www.experts-exchange.com/questions/24298635/XP-Logon-Logoff-Loop.html?cid=236

I have seen this before after removing spyware. I think it has something to do with the userinit entry and/or explorer.exe.  If you do a windows xp repair installation that should get you back on track.  If you prefer to go after the the actual files that are having trouble let me know and we can take that route instead.  You do not lose any data or settings when doing a repair installation so that is a good way to go.  Instructions from MS web site are below:
Configure your computer to start from the CD-ROM drive. For more information about how to do this, refer to your computer's documentation or contact your computer manufacturer. Then insert your Windows XP Setup CD, and restart your computer.
1.
When the Press any key to boot from CD message is displayed on your screen, press a key to start your computer from the Windows XP CD.
2.
Press ENTER when you see the message To setup Windows XP now, and then press ENTER displayed on the Welcome to Setup screen.
3.
Do not choose the option to press R to use the Recovery Console.
4.
In the Windows XP Licensing Agreement, press F8 to agree to the license agreement.
5.
Make sure that your current installation of Windows XP is selected in the box, and then press R to repair Windows XP.
6.
Follow the instructions on the screen to complete Setup.  
Good luck, Rob

Yes, the question I just answered and linked to above -- and dozens more like it -- has the reason: a missing userinit.exe file or a missing reference to it in the registry.

No reason yet to do a Windows Repair install...

Sorry got distracted and see that some others slipped in some posts before I hit submit.  The "going after the actual files" route I mention in my post is what LeeTutor provided for you so you now have both options.  Good luck, Rob.

There are 5 users, I cant log onto any.

Ive tried to

"The registry may be corrupt.  Take a look at this question I just answered a few minutes ago:

https://www.experts-exchange.com/questions/24298635/XP-Logon-Logoff-Loop.html?cid=236" 

Its looking like a windows repair / install is in order.

Any other ideas before I do anyone?

Windows repair ....

Ive never seen a windows repair to give a stable result (approx 8 times on different pcs)
 > compared to a fresh install.

Its always been a waste of time.    What does everyone think of windows repair?

I have used it in this situation a half dozen times or more.  I have done a repair installation well over a dozen times in the last six months.  We repair on average 12 workstations a week.  I find it to be a swift solution to many items, perhaps I am being lazy, but like to get the jobs out the door as quick as possible.  Anyone please feel free to comment.  I take constructive criticism very well.  Good luck, Rob.

Also, check http://thinkinginpixels.com/quick-fixes/fix-windows-xp-log-onlog-off-loop/

worded, any feedback?

A windows repair didnt solve it so I reinsatlled the op sysys

Thanks to everyone who tried to help, but nothing worked appart from nuking it.