Try to REPAIR WINDOWS XP
Well if you antivirus is not picking up anything and you did the above you may want to try and Repair your Windows
Disable all your startup apps before doing this Repair (mind you I have never disabled any of my startup apps when under going a repair)
As always make a backup and/or restore point. You'll need you XP CD and
Follow this guide
Have you serial/product key near by also.Method 2: Reinstall Windows XP by starting your computer from the Windows XP CD
To reinstall Windows XP by starting your computer from the Windows XP CD, follow these steps:
1.Insert the Windows XP CD into your computer's CD-ROM or DVD-ROM drive, and then restart your computer.
2.When the "Press any key to boot from CD" message appears on the screen, press a key to start your computer from the Windows XP CD.
Note Your computer must be configured to start from the CD-ROM or DVD-ROM drive. For more information about how to configure your computer to start from the CD-ROM or DVD-ROM drive, see your computer's documentation or contact your computer manufacturer.
3.You receive the following message on the Welcome to Setup screen that appears:
This portion of the Setup program prepares Microsoft Windows XP to run on your computer:
To setup Windows XP now, press ENTER.
To repair a Windows XP installation using Recovery Console, press R.
To quit Setup without installing Windows XP, press F3.
Press ENTER to set up Windows XP.
4.On the Windows XP Licensing Agreement screen, 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 that appear on the screen to reinstall Windows XP. After you repair Windows XP, you may have to reactivate your copy of Windows XP.
or a direct link on how to just incase is http://support.microsoft.com/default...b;en-us;315341
At the moment can't think of anything else which may cause this problem sorry..