If you want to leave it "as is" linny run ccleaner on it, in the options tab select secure file deletion and change the passes to 2 or 3, it will take longer obviously, you could also try the program gman posted about to search for files you have deleted to make sure they are gone completly, sorry i cant remember what it is but im sure its in the useful free apps section.
Also in ccleaner check all the boxes while the "cleaner" tab is highlighted in both Windows and Applications. Uninstall any software/programs you have put on yourself with either ccleaner or revo uninstaller, revo is probably better to use for this.
After all thats done run the registry cleaner available in ccleaner and do a defrag either with the available one in windows or use Auslogics disk defrag which is free also. Then flog it for as much as possible :)
Clicking Start and run and typing recdisc.exe will also create recovery discs for you once this is finished so you'll basically have recdiscs for a clean system, might be a good selling point too ;)