To see how much space you have left, open the My Computer icon on your desktop (or under the Start menu for XP users) and right-click the drive you are curious about (for most people, this would be their C: drive, but this works with any drive).

Select Properties from the resulting menu. You could also hold down the Alt key and double click the drive you want to open Properties.

You'll get a great looking little pie chart showing you how much disk space is left.

Go ahead... see how big your piece of pie is.