Also worth running is the Disk Cleanup utility (Start > all programs > accessories > System Tools)
If you have System Restore turned on, it may be eating up hard drive space with restore points.
I have also found that the Recycle Bin is set to take up some ridiculous percentage of your drive, like 10% by default or something. I'm not sure if the Recycle Bin reserves
that space, but cleaning out your recycle bin and setting the max recycle bin size lower could be a cheap space saver. Right click on Recycle Bin and click properties, then change the slider down to 5% or less, unless you accidentally delete giant-sized files often. If you click on the tab "Local Disk" it will tell you exactly how much space is being reserved for it.
If your system has hibernation turned on as an option and you don't use it, then turning off hibernation will save you as much space as you have RAM (if you have 1 GB of RAM, Windows blocks off 1 GB of hard drive space. If you have 2-4 GB of RAM, thats a hefty chunk of space you could be using.)
Click Start > Search... and click to search for all files/folders. Then under the "what size is it" section, click specify size.
Enter 50000 (thats about 50 MB, but it wants the number in KB). Now search, and it will show you all of the files bigger than 50 MB. That's a good way to spot any big space hoggers.
If all the space is taken up by mp3s/videos/porn, then well... get yourself another hard drive