"I'm going to completely contradict the other answers posted so far, as I don't believe splitting data/apps/OS between different partitions is really beneficial.
If you have a good backup strategy (you do take backups don't you?), then there really is no need. Windows 7 (and the same applies to other versions of Windows, Mac OS X or Linux), provide you with a directory structure for user data which is perfectly suited to storing all your music, video, office docs, etc. By design it provides security (i.e. you can keep your data private from other users), but you can also share data between…