shows you how to set permissions on a folder (it's for XP, but it's the same on Vista). You can also do it for a drive (right click on the drive, select "Properties" then go to the "Security" tab).
What you need to do is to make entries for the users that you want to deny access to and then tick the "Deny" box in the "Write" row. But be aware that some programs may need to write to the C:\ drive to work properly, so you may need to do a little testing. And make sure you don't change any of the settings in the "Administrators" entry.