1.Most important is Know what you download.
2.Upgrade your browser
Today's browsers have a lot of security controls built in, and these tend to do a fine job of preventing phishing attacks or the automatic downloading of potentially malicious software. Antivirus tools add little to their already robust protections.
If you attempt to access a website that's known to be harmful, most modern web browsers will warn you before the site is displayed on your screen. If you'd like to feel even more secure when you're surfing, consider installing the free McAfee SiteAdvisor tool (http://www.siteadvisor.com), which adds small site rating icons to your search results and a new browser button and optional search box to your browser.
Among them are Eset Online Scanner (http://www.eset.com/online-scanner), Avast Online Scanner (http://onlinescan.avast.com), and TrendMicro Online Scanner (http://housecall.trendmicro.com). All of these allow you to initiate a system scan without installing a permanent antivirus package.
Most of the generalised warnings you hear about virus-carrying email messages are misleading. You can't get a computer virus merely by receiving an email message. You would have to click on a link within the message that downloads a harmful file onto your PC, or you would have to open or run a malicious file attached to a message.
You can add yet another level of security by configuring your email program so that it displays incoming messages as plain text. Doing so will turn off the display of graphics, which, when clicked, may unleash a virus-carrying file. If you use a traditional email client such as Outlook or Outlook Express, add a good spam-blocking like Cloudmark Desktop (http://bit.ly/7zrVeU) to your toolkit, and chances are very good that any potentially dangerous messages will get routed automatically to your Spam folder, where you will never see it.
5.Free and customizable AnitVirus.
I know even doing all this you might not be fully confident without AnitVirus. Titles that meet all of those criteria include Microsoft's Security Essentials (http://www.microsoft.com/security_essentials) - which is free - PC Tools' Spyware Doctor with Antivirus =(http://www.pctools.com/spyware-doctor), and Norton Antivirus 2010 (http://www.symantec.com). Each of these will provide the background scanning and on-demand protection that you might need, and each can be configured not to bug you unless absolutely necessary.