Have you tried booting into safe mode to see if it happens then? If it doesn't, it sounds like it is a startup program or driver that is causing it in normal mode; in that case, type MSCONFIG in the search box off the Start Menu and hit the enter key, then try disabling programs in groups and rebooting to try and narrow down on what is causing the problem.
By the way, I found this page also, which blamed VMware:
[resolved]USB Flash drive doesn't autoplay anymore - Windows Vista Forums
doesn't autoplay anymore, odd wmplayer.exe process when pluggin in drive.