Start-Programs-Accessories-System Tools-Disk- System Restore and create a new Restore point. Name it "Cleanup at TechSpot".
Then Start-Programs-Accessories-System Tools-Disk Cleanup
Click OK to accept C:
Select all Boxes
Then click More Options
Here click System Restore and OK to "Are you sure" and the OK to Run.
As this runs it clears all but the most recent Restore Point but it does one other thing that can contain infested files and a huge amount of disk space.
It clears what is known as Shadow copies which are used by specialized back up programs.
This is if you have the Volume Shadow Copy running which is the default.
After this you may want to do a deeper clean so use the below..
Run CCleaner http://www.ccleaner.com/download/builds (get SLIM at bottom no Yahoo toolbar)
Run twice or more on Cleanup temps, then on left click Registry then Scan for issues also repeat till clean.
Thanks Mike for the useful info. I failed to mention that I wasn't having problems with my computer and my post was to satisfy my curiosity as to what happens when system restore has used up it's alloted space.