1) If your PC has a TV out connection (either s-video or composite) you can do that. You connect your PC TV out to your VHS recorder through an appropriate cable. Then you start playing your avi videos in full screen and hit the record button in your VHS. No special program for this, just a video player software in your PC. I don't know how good results you can get out of this.
2) If your PC doen not have a TV out, you can do it with a sandalone DVD player that is able to play your avi videos. You connect this to the VHS recorder and preceed as normal. I think this will give you a better looking result.
3) Another solution would be to copy your 8mm video tapes directly to VHS. Maybe this is the easiest solution, and probably will give you the same or better result as above.
Anyway, from analogue to digital and then back to analogue is always not a good idea That's why I would first start with (3).