Take a cotton ball, soak it in rubbing alchohol, and hold it inside the computer for a little bit, it will fume out the ants, and probably kill them.
After that, take a little bit of bug repellant, and pit it around the fan and all the vents.
I havent tried this, but i think it will work, also, i think shutting your computer off for a few days will run them out, i heard they like to bite the circuit boards for a little jolt (no lie)