FIRST STEP :
Yeah, First, simply get the code of the battery, and go to computer shop or somewhere where they would sell those circular types to test if it is this that has gone. Its a possibility before worrying that your system has been damaged
If this doesn't fix it, you'll need to use some antivirus software and search for it if its a bug in the system.
It would have gone into the main system, and if this doesn't work, I'm afraid your hard drive would need to be formatted... but at least you can backup data...
One free antivirus scanner which doesn't have a trial version is AVG Antivirus. Download this instead, update and do a scan with it
I DON'T THINK ITS A VIRUS:
I don't think this is a virus of the normal type though but might be a bug AVG will find, like a joke file, since I don't think it would infect EXE files, but I still wouldn't back up any just incase, so only document files, saved files, and music files if you can.
If you have to save some EXE files for instance, run a virus check on them using this before you back up on cd/dvd or on an external drive
BATTERY REMOVAL WONT WORK:
removing the battery wouldn't work since it'd be stored on the hard drive to change it every time it boots up.
You can get all sorts from cracking, and I guess you've learnt the hard way, since anything can be held in the Zip files you download
I hope this helps