That depends on how much you're willing to pay.
Pay nothing: Avira Free Antivirus.
Pay a little: Bitdefender Antivirus Plus.
Pay a moderate amount: Bitdefender Security for XP and Vista.
Pay a lot: Kaspersky Total Security.
The first two don't officially support Vista, but should run on it. Run a trial version of Bitdefender Antivirus Plus to make sure.
I've used AVAST! for years. They have the a great boot scan that can go through your Windows before it boots up. I used it for XP, Vista, Window 7 and 10. It's free to try, is moderately priced but can have options added to it. If I was in high security and wanted the best at any cost, then Kaspersky would be my choice. These two companies have a vast customer base that keeps feeding them virus information everyday. This helps them develop anti-virus updates quickly and installed throughout the day.
I would recommend you to install eset if you are willing to pay a fair amount of money. If not, then install forticlient. They are both very good and they are always in the first 5 antiviruses that find a new virus.