Windows uses several different folders named Temp to store things temporarily. Unfortunately, it is not normally cleared out or emptied. This can add up to a lot of items and space.
In Windows 7 and Vista, type %temp% in the text box above the start globe. That will display all of the contents of all the Temp folders under the currently logged on user. Select all the contents (Ctrl+A or Edit, Select All). Press the Del key on your keyboard. Sometimes it will seem to stop then tell you it can not delete a particular item. If that happens unselect the named item and press the Del key again. This could happen a few times. Some items can not be deleted because they are currently in use.