I just did this last week, and I'm trying a system of partitions that I hope will help to minimize future problems. Of course, with three parts on the same drive, if the drive goes I'm screwed....
First Part is ~50gb. This is where I install windows and programs and nothing else!
Second Part ~100gb. This is where Data that *I* create goes. docs, sheets images etc. etc. in a custom My Documents folder.
Third Part ~100gb. This is mass storage. Downloads, music archive, like it says bulk storage stuff or stuff that will get served to the media player, junk waiting to be burned.
The first partition could conceivably be smaller or larger, depending on how much crap you install. I tend to install a lot of stuff, but then rip out most of it afterwards as programs are big on promise and mostly short on delivery. In my case, this could probably be smaller, even though I have some big pigs installed. Smaller means the image you take for backup is a manageable size. It's also good to segregate windows from anything valuable like data, so that when it finally implodes it doesn't take anything of actual value with it.
The second and third partitions are highly variable and I expect to alter the sizes of these two partitions as time goes on. As the most valuable thing on the hard drive, I want to segregate my data from the OS, and also from the parition in which I hold downloads or junk ready to be burned. It also leaves partition sizes that are easierto backup via imaging.
If you have a Big Assed Music Collection (tm) you will probably want that on a completely different drive rather than just a different partition. I am working on getting it off the main machine altogether and onto a dedicated box. In the menatime, a partition works for me.
Last thing, I want to go back to the custom Documents folder. I *always* move this to a short name folder somewhere, because the length of the path to the doc folder inside your windows profile is stupidly long - you'd be surprised how many zip programs will choke on the length of this dog pathname, and as many backup programs use zip, I see alot of failure in that areas as well. To boot, a lot of the failures occur when you attempt to restore rather there when attempting to do a backup, so be aware.
For me the folder is MyDocs and is in the root of the document partition.
Good luck in wrangling your Windows. Lord knows anyone of us could do a better job than the way the thing comes packaged