Can you tell me what file type they are originally? You'll want to save compression (to any codec) as the LAST step. The quality suffers greatly if you continuously re-compress. It's best to stick with .avi as long as possible.
I have used avi fixers before; I can't remember the names of any of them. You can try to google what you're looking for. I came up with
www.fixvideo.com as the first hit. Basically you need to rebuild the index. You might also try Vdub (or VirtualDub) from
www.virtualdub.org.
I'd seriously consider upgrading that processor ... it's not as expensive as you think.