Before that, running the Windows built-in cleaner should always be the first step. It's dead easy; just go to "This PC" ("My computer" on XP/7, "Computer" on Vista) and then right click on the drive (normally "C:") and select properties.

When the properties box comes up, click on the "Disk Cleanup" button on the right side next to the space pie chart.

When it comes up, make sure all the options are checked. If it finds a fair bit of space to clean up, click OK.

If it doesn't find a lot of space to free up, try the "clean up system files" button; that will often find more to clean up

Under the system files mode, "More Options" tab will also appear, and using the lower "Clean up" button under "System restore and shadow copies" will often (but not always) free up a lot of space.

Lastly, by default Windows will use a huge pagefile, and have a hibernate file; minimizing the page file size (around 1gb, for example) and removing the hibernate file will both typically save a lot of space (especially on a machine with a lot of memory.)