I personally use AVS Video Converter, It costs about $39 but it's completely worth it. It will convert from just about any format to just about any format. I used it to convert all my DVD rips from .avi to .mp4's for
my iPod. It has full WMV support so it can convert your m4v files for windows media player with ease.
Here's some of the features from their site:
Read: HD Video (inc. Blu-ray video, AVCHD, MPEG-2 HD and WMV HD), AVI (DivX, Xvid, etc.), DV AVI, MP4 (inc. Sony PSP, Apple iPod and Archos), M4V, WMV, 3GP, 3G2, QuickTime (MOV, QT), DVD, VOB, VRO, MPEG-1, 2, 4, TOD, MOD, MPG, DAT, VCD, SVCD, Real Video (RM, RMVB), ASF, ASX, MJPEG, H.263, H.264, DVR-MS, MKV, OGM, FLV, AMV, MTV, TS, M2TS, M2T, TMS, DPG, NSV, FLI, FLC, CDG.
So I recommend you this tool, you can free download to have a try,
http://www.avs4you.com/AVS-Video-Converter.aspx
If you stick to freeware, SUPER would be your best choice, it's an general free video converter
http://www.erightsoft.com/super.html
Hope it is helpful
And this fourm is for mac os only, maybe you can find windows answer from videohelp