The point is if you can run them from within Win7 now then the presence or absence of WinXP is immaterial as they're self contained and running without reference to the OS they were installed under. They weren't installed under Win7 so Win7 can have no "knowledge" of file locations and settings. The programs themselves are providing that information when they run.
For starters look at the directory they're installed in (as in where does the shortcut you run them with point) - they most likely have their own directory that contains all the files needed. It's much like the current day "portable" installations where you can run software "installed" on a flash drive.
I doubt leaving WinXP intact but unused will pose much of a danger because trying to run anything from it under Win7 will fail (wrong registry, wrong files and everything in the wrong place).