I know for sure Simple File Sharing isn't required, 'cause I have that turned off on all my XP Pro machines and my cruzer micros used to autorun U3 just fine. But I also recently disabled autorun on everything, CD/DVD drives and removable (thanks to the malware on Sony CDs). To make loading U3 easier I added it to the Removable Disk dialog using My Computer->Autoplay->Handlers in the powertoy TweakUI for XP... so when the computer scans the thumbdrive and asks what I want to do I just choose Load U3 and click OK.