My bet is system recovery files. They can all be deleted, IF you are certain your system is good and uninfected. I think the procedure is just disable system restore, close down. Reboot and set system restore on again.
By the way, a drive like 200Gb and absolutely a 20Gb like the original poster, should most definitely be accompanied by a cheap, cheerful and very large USB drive. What - £50 for 300Gb ?? Then you can just turn off system restore for good, and before installing anything do proper system images on a routine basis to the USB backup drive. One day you will be on your knees blessing that approach....