only for the x64 version of vista they made it so you couldnt install any drivers that arent Digitally Signed. Some programs like AtiTool will come up as unverified, and wont let you install it. There is no way to disable this through system options, you have to enter a cmd prompt.
http://news.softpedia.com/news/Disable-Driver-Signing-in-x64-Windows-Vista-53491.shtml
http://news.softpedia.com/news/Disable-Driver-Signing-in-x64-Windows-Vista-53491.shtml