Assuming the answer to my previous post is Yes, its time for a new approach!
I have exhausted all the possible causes that I know of, then with your version of XP being different to mine, it was time to talk with a couple of mates, who agree that the first port of call is spyware.
It could be a program that is not working properly, programmed improperly or spyware/viruses that have not been found by the standard route.
This is what I think you should try
Create a directory on your hardrive and call it ‘Hijackthis’. If you do not do this, you will not be able to use the backup/restore features.
Download HijackThis from this
link.
Save this file into the directory you made previously and then run the program named hijackthis.exe.
When the program opens click on the Config button, then click on the Misc Tools button, and click on the Check for update online button. When it completes checking/applying updates press the back button.
Now click on the Scan button and when it is finished click on the Save Log button. A Notepad window will open with the contents of this log. Click on Edit then click on Select all. Then click on Edit and then Click on Copy, now paste it into a reply post here so I can look at it for you.
Don’t delete anything that HijackThis finds just yet.