I've been editing for a long time now, and I'm pretty familiar with different reasons for the game crashing once it loads up... this is wierd, though.

The level works fine on my other computer, but on this one the game crashes just after the loading bar hits the end.

It's not specific to this level, either. Every time I try to play something I've made it crashes. I can play the original SP levels fine. Weird, eh?
[edit]
Actually, I can't even get SP to start. The game crashes right after it gets done loading, no matter what it is loading. Any ideas?

Corrupt resource files? I say copy the res1.gob and res2hi.gob off the disc and put it in your JK/resource folder again.

That was a good idea, but it didn't work.

Open JK, select player, then go into Setup.

Verify your video settings. Messed up video settings is the most common reason that I have seen for JK crashing on every map.

Thankyou SO much ZeqMacaw! For some reason my game was set to the lowest resolution and it was messing things up.