There are a lot of products out there, and there is a lot of scaremongering amongst the big companies making the products. Anti-virus and firewalls - harped as being the saviour of the internet. But the only experience I have had with paid ones is bad. I once brought norton because I was sick of getting viral (bad viral 
but within 2 weeks it would not run (after long setup) because it thought I was using an illegal copy - After I had brought it??
Either way I can happily say I have gone 2+ years without a single virus ‘oh oh’ moment. I could put this down to improving windows updates or cleaner internet - but I don’t. Here’s how I subvert viral disaster in three easy steps.
- Adaware - The first in your arsenal of seriously good free bug killers. Click ‘Download adAware for free’ -I havent ever tried the paid versions - normal one does it for me! Download it - update it and then run a full scan - I defy you not to find anything!
- Avast - Secondly go with avast. For resident protection (monitors all transfer, opening and editing of files on your pc) self updating all round goodness. This is your last barrier - if you manage to get a bug of any severity onto your pc this will find it and lock it up (quarantine it.) Again if you download the ‘home’ version its free to run - obviously if you wanted to use it in your workplace you would have to pay.
- Spybot (Search and destroy) - Finally spybot - a seriously great free program that will find all the little sneaky bugs that are watching your computer. Has great features like immunisation against threats, easily updated and if you change to advanced menu (at top somewhere) it includes a secure shredder - which will make your files 70% unreadable after you delete them! Again free and worth doing once a month or everytime your computer starts lagging.
By the way I have no affiliation with these products - they are free, I have just used them myself for years and would recommend them to anyone. Not sure about vista compatibility by the way…