I've been getting that lately, myself; I can render to .avi once in a session, but the second time gives me an error message. Personally, I blame Microsoft; I think Bill Gates is trying to subtly encourage me to upgrade to WinXP by making Win98 increasingly unreliable.

Render to image sequence (.bmp) instead, then use a video editing utility like VirtualDub or JASC Animation Shop to assemble the images into an .avi.