Have you tried to start the Safe Mode variant of it, or reinstall it? It could be one of those random Windows errors. My comp for instance starts to think for no reason sometimes when I'm browsing in explorer. My "Help and Support" button mysteriously stopped to work, as well as my F-Secure AV program.
Anyway S?ldner is a good game (if you've got good ping), the bad thing is that one person both steers and handles the guns in the car, but that's ok. THere are many odd vehicles like Tractor and Lada (nice idea, but undetalied, ok it's a beta). Will play it when I get more time.