So, I hear rendering takes a long time. Can you just leave your computer, like switch users on a win XP while its doing it without disturbing?

Short Answer: no

Long Answer: no . . . . in windows . . . rendering will take 100% of your CPU . . . so it will pretty much make your computer unusable. Not to mention RAM requirements of a render (if the detail is high).

If you have Dual core CPU, theoretically you should be able to do it alright.

In WindowsXP though, there is a function in the Task Manager which will allow you to set process priority. Set blender.exe to a lower priority if you want to you your computer and have it reasonably responsive. This will increase rendertimes though.