If I were you I would re-dl M&B, then reinstall windows, and after installing all the drivers etc. install M&B again and see if it runs. (some files may have gotten corrupt during the dl) If it runs means some software somewhere glitched, if it doesn't then I have no idea. ::*(
(I have a spare FX5200 so I will test and c if it causes any probs, but I doubt it would be a gfx card problem..)
Another thing which I would try first though is pulling the power from your cable modem (so that you can off all the firewalls etc without compromising your system security) - off all the firewalls you have access to, open task manager, tab 'process', find 'explorer.exe', right click and select 'end all process tree'. This would end the process of your explorer and most of the other extra 'services' which are un-needed, leaving you with the task manager. (You would notice your 'processes count drop to below 20). It is not a bullet-proof method, but it usually shuts down most of spyware / trojan processes, amongs other things.
*note- you will recieve an 'access denied' message, don't worry about it, its basically refusing to shut down one process which is critical. It does not affect the rest. Also make sure you select 'end process tree ' and not simply end process
Go to file, new task, and type <drive letter>:\<M&B folder>\mount&blade.exe - this should start M&B direct without explorer, or hopefully other conflicting programs which run in the background. You can start the explorer by 'new task' and typing 'explorer.exe' (without any drive letter or \).