You could try NOD32 as they have been vista compliant since before rtm vista and is the best version of any security software currently on the market.
http://www.eset.co.uk they do have a 30 day trial of both their AV and Security Suite solutions. Not only is it fast and extreemly thorough it is the only security software I have seen try to up date itself every hour you are online to help prevent infection - this is configured as default.
I now only sell their Security Software despite being a Symantec Partner ( I do sell their other software though)
.l
Dont just take my work for it, check it out.
And with regards to whether to pay or not - I have seen and regularly have to repair machines using free products - the product isnt very free when I have had to add my bill "cos" the program didnt do the job properly!
