[gerbilcannon]

Defrag hdd, if you're using an iso delete it and make a new one, if you aren't using an iso, give that a try, and don't forget to clean your cd as well, could be that one errant nearly invisible fingerprint that is messing everything up. Just had that happen with my Planescape: Torment cds. And beyond those suggestions, get new cds then, that's all I can think of sorry!