That crappy error... what
crappy error? Is it the one in which WMP tells you that you're missing codecs?
Regardless, first thing you do is get CCCP
and start using MPC-HC instead of WMP, next thing you reinstall avisynth, making sure it's the latest stable version (2.5.8) and then you go and manually reinstall the lossless codec you used to make those AVI files, which I assume is one of Lagarith, Huffyuv, or UTVideo. In case you were using fake avi files, I'd suggest switching to lossless clips editing, and I personally recommend the last of the three codecs I mentioned for the purpose.