This is how I would do it.

1. Open the original WMV file in Windows Movie Maker

2. Trim/split it as you require

3. 'Save to my computer' using appropriate save settings for your target bandwidth - the result will be a WMV file

If Windows Movie Maker's export options are not flexible enough, try saving it to DV-AVI format. Encode the resulting avi file using Window Media Encoder (free download).

I suppose the quality of the resulting WMV file could never be better than the original source file. What quality was this?

Also, what was it's frame rate? This should determine whether to choose an NTSC or PAL encoding profile.

Hope this helps

Simon

PS - If all you want to do is trim the original WMV file (as opposed to rearranging it in any way), Windows Media Encoder also comes with a utility called Windows Media File Editor. This will allow you to 'top and tail' a WMV file without affecting it's quality.