safe to remove second OS on a second hard drive

well as of tomoro i will have 3 harddrives all with their own OS xp-vista-win7

my old pc that contained my vista drive was bought from ebay, and of course once an update failed i needed to reinstall the os, of course as it was from ebay i couldnt reinstall, so i got my old pc's hard drive that had an old version of xp on it, so now both the hard drives contain data that i want to keep, however i wanna free up room on my vista drive, and the best way for me to do that will be to remove vista as its doing nothing and is corrupt anyways

however i would like to know if its possible to remove vista, by deleting its "windows" section in the hard drive or by uninstalling it but still be able to get to all the files currently on the hard drive, im aware it seems an odd question, i know you dont need an os on each harddrive for it to work however im uncertain if the files are somehow attached to the os they was saved too, so for example deleating vista (i would still have an os (win7) would render all my files saved on my vista harddrive unusable as i was reading a similar post of this on win98 and the clever pc people at the time said this sorta thing wouldnt work, however of course its been nearly 10 years since that post, and the coding has changed an awful lot as well as a lot more different OS' companies around

my win7 hdd will be new and contain win7 and i intend to install my games to that, then i will have a second harddrive (my vista drive) for media, thus why i wanna remove vista from it, my 3rd drive (xp drive) will remain in my old pc incase i dont like win7 although ive heard its got the proformance of xp but looks simliar to vista

 

I can't see any reason why you couldn't remove Vista - but then I would have said the same about W98. As far as I'm aware, the user files are not attached to the OS at all (although some games and such might only be compatible with certain OSs).
Anyway, if you're in doubt, are you able to copy the files you want on the Vista HD (hard drive) onto one of the other HDs (or onto a CD if there isn't enough room), just in case? That would be a good idea anyway, as things do go wrong sometimes.
If you can uninstall Vista, that would be the better approach, but I can't see how you could do that if it's not working. You might be best to just copy everything you want, then reformat the disc and start fresh.