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 other users if you prefer.
As the other answers state, you might have to re-install windows, every now and again because it goes pear shaped, but if this happens, why not just restore from your most recent backup? You state you are running Windows 7, and if you are using the native backup tools, your backup will include the OS, apps and data. So a restore will take care of everything.
I wouldn't be happy reinstalling Windows on a disk that contained data I cared about, even if the data was on another partition, and what is to say that if your OS is hosed, that your data can be trusted anyway e.g. if you suffer a virus infection or a system compromise, then can you really trust your data and only reinstall the OS?
The other drawback to splitting partitions between data, apps and OS, is that you have to guess what size to allocated to each. When one fills up and others are empty, you will regret doing this.
On the contrary however, if you are looking for ultimate performance, I would install the OS on a fast SSD disk, and keep my data on a cheaper conventional disk, however you suggested in a comment that you have a laptop, so this probably isn't an option for you.