if its a virus, be VERY careful on the data backup. To often I've seen people format a system due to a virus and then put there "backup files" on again. Well, those files where infected so all they were doing is re-introducing the virus back onto the system. Back it up and scan it before restoring the files. Also, sounds like you have some kind of stacker/ duplication virus. Basically it adds a few bites to each file and starts to duplicate. Eats HDD space. If you dont have the latest definitions I'd goto
and run a scan. Its free and it also cleans the infected system. Reason they get away with this is you can run a virus scan and cleanup for free, but it doesn't protect you from future infections, u just have to go back to the site from time to time and run it again, but there is no preventive measurments like one you would purchase.... its also updated often.