do you mean your hard drive i nearly full?
if so firstly click start and then control panel, then select add remove programs and look through the list and remove any programs that you know you no longer use (if not sure ask what it is here or google it)
then download ccleaner
http://www.ccleaner.com/download run it and firstly analyze then clear that will clear all your old temp files etc
also if you have a lot of thing like pictures or music on your computer consider backing them up to CD and removing them from the actual computer.