For a start, you could clean out all your temporary files and see what space this releases.
Go into Windows Explorer and find the Temp file. It will normally be C:\Windows\Temp. Now clear out all the files listed as temporary files (normally with a .tmp extension).
You can also do the same in C\Windows\Internet Logs.
Then do a search for any further temp or tmp files and get rid of those as well.
If no "house-keeping" has been carried out for a while, you will be surprised at how many of these files accumulate, and the amount of space they use.