This can take some time- hope you're up to it.
This has to be done before Windows loads, so go through start-up from a total shut down so it clears out the buffers, and starts with the operating system.
One of these buttons should be the internal System Recovery. I will take a somewhat educated guess and say F11, but it may be F9 or F10. If that doesn't work try ESC. Press and hold until the OS screen appears. You should get several options. Going this route will only restore the system at whatever point the recovery was created, so this is significant.
If you don't want to do that, one of those buttons should be a Diagnostic program that you might be able to isolate the bug with. Another is Set-up, which might be another way to isolate the contaminated application.
The idea is to stop the virus from loading with XP without the virus blocking the effort. If you can get some things to function, then you could load a cleanup program and get rid of it.
Just my 2 cents worth. I'm sure the resident computer gurus have a better way...