Interesting way to work around the profile corruption bugs in Titan Quest - setup the volume to use the Previous Copies client in Vista. Using the ForceWare 88.61 drivers with build 5456 and the 3665 build version of the SoundMax XP drivers for my ASUS P4C800-E Deluxe makes Titan Quest work really well with my GeForce 6600GT.
I had some Father's Day money left over on my gift card from Best Buy and some birthday money so I picked up Titan Quest on DVD this week during vacation. The new Titan Quest v1.08 just came out so that will hopefully fix many of the crash bugs but ironically, I haven't run into ANY crash bugs under XP or Vista on my system, which seems to be rare, apparently.
Performance seems to be the same under XP and Vista, although my Vista setup is much like XP. Classic skin, User Access Protection turned off, all bells and whistles turned off. Overall the system feels snappier under Vista, but I think it is due to better use of threads and hyperthreading.