VirtualDub works quite well at appending AVI files together; it'll even import MPEG-1 files for conversion to AVI.

TMPGEnc will allow you to stitch MPEGs together.

Both applications are freeware, and I've used them for years. I'd be happy to field questions on their operation over in the much-neglected Director's Cut forum, if anyone needs help.

Oh, and you can use a lossless-compression codec (like Huffyuv) if you want to maintain perfect image quality while editing, but don't feel like working with stills.