If you have W7 and XP on the same drive, in separate partitions, or if you have them on different drives and are using the BOOT menu provided with W7, then I think you'll find W7 will tend to do strange things to XP.
I can't be certain if this was especially a problem with XP, but it happens with Windows and some versions of Linux.
The only safe way to use both OSs is to keep both of separate drives and switch over when BOOTing using the BOOT menu itself.
While most BOOT versions say to press F* to enter setup, this has generally been over written. Press F5