What Operating System are you running? If it's Vista, that could be part of the issue.
Are you logged into user account with administrator priveledges? If not, that could be the problem. Even if you think you are, you should double check. Your roommate may have changed that while quarantining files on you.
As mentioned, install to another location, such as a mIRC folder on the desktop and see if that helps. Maybe also try temporarily disabling any antivirus and anti-spyware software to see if that helps.
Beyond trying to get mIRC running again, I'd recommend telling your roommate not to change anything on your computer for any reason. It's never a good idea to let others change things. Letting them use your computer is one thing, but it's still your computer and a roommate should never be changing things on you.
