If there isn't anything too valuable on your computers hard drive I would go ahead and reinstall. If there is I would recommend getting an external HDD or USB thumb drive and putting it on that. At this point an antivirus might be blocked from being installed, so I would say wipe the HDD. You should also change all passwords of ALL accounts, because it is possible that this is a key logger.
(DO NOT use the infected computer to change passwords, and make sure you change email password first.)
I have had many accounts stolen by icky viruses so take this seriously, you could lose an account or worse, a lot of money.
Recommended antivirus software for once you reinstall is Avast. It does the job just fine and it is free.
Good luck
(Edit: also make sure that this a virus and not some sick joke your friends pulled on you. It could have just been a popup joke or something.)