My system:
AMD turion x2 (2.0GHz)
2GB RAM
ATI mobility Radeon 3650
Windows 7 x64
The game works great on my desktop. But on this laptop, I get the "Cannot find the main class: com.threerings.projectx.client.ProjectXApp." error. I tried to update java, and still no use.
ps. I'm using the steam version
Try verifying the game cache or re-downloading. It sounds like you didn't get all of the game downloaded through Steam.