Let me start this message by saying that PC security is your concern and responsibility.
My biggest problem with vista was the messages that said "xxx is trying to activate is that ok"....you click yes and it asks if you are sure. I would sit there thinking "I just clicked the darn thing so yeah, i'm sure". User account control (UAC) is one of the biggest causes of frustration among users, but there's a simple solution...turn it off.
If you have had prior versions of windows and never had a virus, or at least never had one that wasn't caught by download managers or antivirus, and have a semi decent firewall then just turn it off.
To do this, just go into the user accounts section of Vista control panel. There you will see all of the user accounts. Yours should be set to administrator, If not, and you are the administrator message me. Inside of there you will see an option to turn user account control on or off. Click that and shut it off....done.
some notes on UAC
http://technet2.microsoft.com/WindowsVista/en/library/00d04415-2b2f-422c-b70e-b18ff918c2811033.mspx?mfr=true
brad
PS I spent some time studying Vista after getting my laptop, and discovered a few tricks to help it along