Well, 6.40 didn't work for me either. In fact, MED crashed before the engine preview window even finished loading the model this time.

I think I'm going to have to reinstall my operating system (though I'm going to try a Windows Repair first to see if I can get away with not having to re-install all my applications too...).

You may also check this bugs list before doing so.

I didn't see anything that looked applicable on the list...

However, I appear to have fixed it.

After spending about 4 hours backing up all my data in preparation for re-installing the entire operating system and all my files, on a lark I decided to try re-applying Windows XP's SP2 since it didn't look like the Windows XP CD was going to offer me the choice of simply repairing my installation. So I ran the SP2, rebooted, tried the engine preview in MED AND IT WORKED!!! Also, WED works properly as well.

All I can say is that the events leading up to this problem began with ZoneAlarm's TrueVector service crapping out, after which I uninstalled it and then installed Comodo firewall, Avast! antivirus, and Spyware Terminator (I've since unsintalled all 3), and even then MED & WED appeared to be working fine until the next morning Avast! ran a pre-boot system scan and picked up a supposed virus among the Java files in the Local Settings/(whatever) subdirectory and I opted to delete it. It was a day later that I again ran 3DGS and experienced the above-mentioned problem.

So maybe this info will help someone else avoid the same thing. But if you do experience the same issue, try re-installing XP's SP2 (if you're running XP SP2+) and see if that fixes it.

Just an idea but are you using 6.5? because from my experience 6.5 has caused me so many bugs just like the on you describe, in fact its annoyed me so much im still using 6 now, perhaps this could help too.