Buy a PC/ install Windows via Bootcamp! I know that sounds harsh, but working with these PC formats on Mac just isn't smart. I don't think you will be able to solve this in any other way. It's the most undesirable combination imaginable - PC-centric container format (AVI) with a PC-centric CoDec (Xvid) run through a 32bit middleware (Perian/ Quicktime) into a 64bit Program (AE CS5) on a Mac. See how that doesn't make any sense at all? Again, it is simply an atrocious workflow and problems are inevitable. The only real other alternative would be for whoever is supplying the files to use a more standard format like H.264.
contact the supplier of the clip and insist on a more reasonable, lower common denominator file format.
If you can see this file in QT, you should be able to save it out of QTPro, if you have it, to a more useful fomrat.
Do you have Final Cut Studio which includes the program Compressor? That's a great program to convert files. I know you said you don't have a lot of time to change formats, but it's fairly quick and you can queue multiple files to convert so you can have them all convert over night or something and ready to edit the next day. Another program, which is free, that you could look into is "MPEG Streamclip"...I'm not sure if it likes .avi's but it is a free video/audio conversion program that won't add a watermark. Just a couple ideas. Hope it helps.