1) Dont confuse memory and hard drive space
2) Do the disk clean up thing (cleanmgr from the run prompt)
3) Check the properties of the folders in the My Docs and Prog Files folders. Find any big ones? well, if its in M Docs, ask yourself if you can delete it, or if Prog Files, uninstall it if not essential
4) Do a search on files >10mb (for example, more if you want). Delete the non essential big ones.