I'm not using WinAmp...I think Media Player is just placing an incorrect (almost random, but always too short) track length in the tag (if that's the correct term). I don't see why, most of the time they are fine.
So, the track length appears wrong in anything I try to play the file back in (Media Player, portable MP3 player etc.). However, when the track reaches the (incorrect) total track length, it just continues playing as normal. So really just the time display and progress bar look incorrect, nothing is wrong with the audio.
I could stop using wma's but a) I like them and b) I'm not convinced the problem would go away even if I were using MP3's or another format.
Thanks for your reply tho