Anilrgowda

A computer virus can be much worse than a physical sniffle. A particularly creative and nasty virus, Trojan horse, or worm can harvest your e-mail inbox so an unscrupulous e-marketer can spam your friends. That's embarrassing, but some viruses may damage your hard drive and data. A Trojan horse program could conscript your PC into an army of zombies used to attack a server somewhere, possibly at some future date.
The fact is, every PC needs security protection that includes antivirus software today. So, if you've loaded up Vista (or plan to), you need to be aware of these particular security issues with Microsoft's newest operating system. Microsoft promotes Vista as among its most secure products, but its handling of antivirus functions may surprise you

Source:PC World - Protect Vista PCs from Viruses