Windows does a lot of work keeping up with the programs you run, the web sites and the content associated with them, your documents, spreadsheets, and graphics work. In the process of doing all those things Windows, and those various programs you run, create temporary files, cache files, and a multitude of other files which can clutter the hard disk, consume space, and slow your computer.
There is a useful utility called "Disk Cleanup" that comes with windows, but is inconveniently hidden away where most users never notice it. You'll find it under All Programs -> Accessories -> Sysstem Tools -> Disk Cleanup. It may run for a while, so you may want to run it before heading to lunch or going to bed.
When it first loads it may ask you for the drive to clean. If it does, select C:. Disk cleanup will spend some time analyzing your disk and then present you with a window containing several checkboxes. Select them all and click 'Start'. At this point the computer could spend a few minutes or an hour clearing off all the litter left on your computer. But, when it's done, you'll have reclaimed that wasted space on your drive and will likely gain some performance improvements. If there is more than one drive in your computer you can run it for each of these, but 'C:' is usually where you get the big improvements.
A great thing to do after Disk Cleanup is to Defragment your hard drive. This will neatly reorganize the remaining files for maximum efficiency.