Fortunately lecture content is generally pretty easy to encode.
You’ll need something better than Flip4Mac to get the bitrate down low enough. Expression Encoder 4 SP2 is free for WMV/VC-1 encoding, and quite good at this kind of stuff. What’s the minimum version of Windows you need to support? Assuming XP SP2 (with Windows Media Player 9), I’d look at doing something like:
Audio:
WMA VBR Unconstrained at 48 Kbps if there’s occasional music or sound effects.
If it’s just voice you care about, you could use WMA Voice at 20 Kbps.
Video:
VC-1 Main VBR Constrained at 512×288
Average bitrate of whatever’s left after audio
Keyframe interval of 10 seconds
Turn on Denoise Filter
Turn off Closed GOP
And if you’ve got CPU time to burn, these options are slower-but-better:
Motion Chroma Search=Adaptive True Chroma
Motion Match Method=Adaptive
If you can afford