1) No antivirus will keep you safe from everything. Just like the flu shot won't keep you from all forms of sickness.
2) Keep you computer up to date. When you get messages from your systray (the area by your clock) that tells you to update Windows, Java, Flash or Adobe, DO IT. These updates are to fix security problems that make you vulnerable to viruses
3) Do not accept any upgrades that pop up on the internet. Frequently these are fakes that will put a virus on your machine.
4) Don't click on links in emails unless you are ABSOLUTELY certain what it is. Fake emails from lots of shippers, retailers and banks contain links that will infect you if you click on them.
5) Unless you know them, be suspicious of any tech support person that offers to get on your machine for free to look. ESPECIALY if you found a support line by a search engine.
If you do get a virus the faster you act the better off you are. Just like your being sick, if you try to ignore it you will only get worse. If you don't know what to do shut off the computer before you talk to someone.