I have seen odd things in Vista related to performance, Typically I have been uninstalling the offending software. Rick found a link to UAC
User Account Control and Application Slowdowns - Rick Strahl's Web Log
I personally don't think so highly of UAC, generally your typical user is just going to click allow because all of us "Bad Software Developers" will teach them to do so. My gut feeling is, I am a uber user and thus find it far more annoying then lets say my Mother would.(Although the UAC prompt is almost as scary as an anti spyware/virus prompt. So maybe the system will never get turned on, which would be safe. Right?) If we as developers work in our sand box, then UAC probably won't be as big an issue as Apple's commercial would like to make it.