Elfwyn, K-Money, WarrantOfficer
You now have access to the beta group.
Elfwyn;
If you Alt-Tab, can you switch to the game, or does that reveal any anti-virus or firewall messages, etc, that you may not have noticed before?
I'd try rebooting before you do too much other troubleshooting.
Have you tried right clicking on the executable, selecting Properties, switching to the Compatibility tab and (for Windows 7) disabling desktop composition, etc?
Are you shutting down all non-essential programs before starting the game?
Do you have this problem with any other games?
WarrantOfficer;
Did you try lowering the game resolution?
What are you system specs, compared to the minimum / recommended?