Well, I do not know if this will work in XP..*cough*...but it works in win 2k pro,
I use 2 little programs, one that is old, and one not so old.
These programs are referred to as "system wipers".
Actualy, the old one is named "Systemwiper.exe" and the other I picked up from
a freebie site, called "CCleaner".
Now the programs are not important as what they do. They simply go thru such common things as temp files, internet cookie cr*p and the like.
What tends to show up, is what CANNOT be deleted. This would be any temp file that is currently in use. If you have a simple wiper that performs this task, use it. See if a file shows up that cannot be deleted.
If this is the case, it gives you something to start with. You may be able to google the name of the temp file and gain some info as to what it belongs to.
With that in hand, the next thing I would do is look thru the registry to see if you can find this culprit.
Again, I use an oldie but goodie for this called RegCleaner.exe. This is a pretty powerful utility that will show you a whole host of info.
I tend to use older programs because the new stuff does not know how to handle them, and thus is more likely to show up.
With the proper entries either deleted or renamed in the registry, (YES, BACK UP YOUR REGISTRY), and with the un-deletable files marked for deletion on reboot, reboot your system and see if this does the trick. This is my recipe anyway, it has worked for me when an unknown item is running, and does not make itself apparent.
Remember, messing with your system registry can be dangerous. ALWAYS back it up before making any changes. If it goes fubar on you, you should be able to boot to safe mode and restore the backup.
Sorry if this is a bit long winded, I am no expert by any stretch. I just know what works for me.