I've come across this rare issue, I have a friend that uses some software that is particularly slow in Windows 7, the best compatibility is in Windows XP, I told him to make a partition and have both OSes in the computer, at which he replied: "Could I have a different OS for each user?", it seemed like a good question to me, so is that possible?, suppose that I login and it boots me to Vista, then I loggout and Linus Torvalds comes and logins and boots him into Ubuntu, he gets bored and logs out, afterwards my friend comes and logs in, booting into XP, summarizing:
The operating system that will be loaded will depend on the user that logs in.