Can't select things in Worldcraft? (Half-Life map editor)

I cant select blocks in the 3d window in worldcraft in windows 2000... it worked fine in windows 98, but in windows 2000 I cant, I heard of this problem before, but it didn't apply to me at the time, because I wasn't using Win2k...

Any Fixes anyone?

Specs:
AMD Athlon 750
Asus K7V
Voodoo 3 3000
256 mb RAM