It could be any number of problems.

Maybe you don't have enough free space on your hard drive.

Maybe you're running out of memory.

Maybe the drivers for your graphics card need to be updated.

Maybe the latest graphics card drivers broke something about the game.

Maybe your hard drive needs to be defragmented.

The 1.x client is awful as far as its programming is concerned. Even a fragmented file in the game directory can muck the whole thing up. I've had to uninstall and reinstall the game twice in as many months because it simply stopped working properly. On the other hand, there are lots of people who've been playing since launch and never had a single problem.

All of us live in hope that 2.0 will solve all our problems.