Windows Media Player Problems

How come I can’t fast forward in my MPEGs? Sometimes I can after I watch through the whole thing once, but it doesn’t work in a lot of videos. REalplayer won’t play it cuz it says something bout MPEG headers or something, i cant remember. Any help? Any other free MPEG players?
Thanks!

Could be a corrupt file structure. If you truncated a download, the key frames could be messed up–basically, the bits of the file that normally allow you to jump to certain points are either missing or corrupted. If you were bored enough, you could try re-muxing it with VirtualDub or some such tool.