You have to find the manufacturer and model number from whatever sticker is on the webcam. If you cant find one you are stuck.
Its also possible that an older webcam may not have drivers for Vista/Win7. I got a cheap Pixxo last xmas that has XP drivers but no Vista. I looked at Frys and that same pixxo seems to be still on sale for $10. The $20 and $25 pixxo webcams say they have will run with vista but the $10 and $15 dont.