Most likely a codec conflict. I disapprove personally of using fansubs as source material... 'cause they're ugly... but here's a way that should fix it:
Uninstall ALL codecs and codec packs that are shown in the Add/Remove Programs list. To access this list, go to Start-->Control Panel.
Now, install these codecs (assuming you are only wanting to import straight into WMM without using any other programs like VirtualDub):
CCCP:
http://cccp-project.net
That should fix it. Normally I'd suggest more... but yeah... that's all you
need.