Since yesterday, whenever I play VMK, my system just crashes after 30 minutes. I've tried clearing my Cache and cookies but it doesn't seem to work. The only thing running in the background is Norton, could it have caused this problem?

No, I have Norton as well, it doesn't affect it.

Can't help you though, it's probably just your computer. Does it feel slower than usual?

You can try:
Click Start
Control Panel
Performance and Maintenance
Free up space on your hard disk

A popup will come up for Disk Cleanup
Make sure you check Temporary Internet Files
Then click ok.
Click yes.

You can also unplug your modem for a few minutes.

Also you may want to do a defrag.