I can report that mineral spirits works very well for removing rubberized-undercoating from the inside of a computer case. It's just a matter pouring some on, letting it sit for a bit, and then wiping off the gunky mess with paper towels.
Of course, the smarter thing to do is don't spray rubberized undercoating on the inside of your case in the first place. If a co-worker plants the idea in your head that it might be a good way to muffle the noise your machine makes, because it seems to work well for cars, DON'T FALL FOR IT! :)
Rubberized-undercoating is really rubberized asphalt - a tar-like substance that makes an ugly mess, and is nasty stuff to have inside your case. Sure it quiets things down a bit, but it never really dries/solidifies, and stays semi-sticky, getting worse as things get warmer (and attracts dust).
This message has been brought to you as a public service from CTADSS (the Coalition To Avoid Doing Stupid Stuff).