First of all, if you're using Windows 7 with AVG anti-virus: we have a problem with that combination. The problem is in AVG itself, and there's no reasonable way to resolve it on our end. Unfortunately, exceptions or disabling the anti-virus won't work. The only solution is to replace AVG with something else.
If that's not the issue, but you're still using Windows, please update to the latest service pack and update/re-install your GPU drivers, then try again.
If you're problem is still not resolved, or you're using a Mac or Linux, please write us reporting the exact details of the problem at helpdesk.