Hi there,
Did you install some new software in that time?
Do you have antivirus protection? Viruses can do that kind of damage...
What kind of hdd do you have and do you use any cleaning and/or defragmentation software?
You should check your graphic card profiles (I'm not sure where to find them because I haven't been using ATI graphic cards since 4 series) and see if your AA settings aren't set too high. My friend had some similar problems while playing Darksiders on his ATI 4870. Game was running smoothly and suddenly some strange issues started to happen but when he decreased his AA value in graphic card profile, all problems disappeared...