how can we protect our computer from virus ?

For average users running Windows 2000, XP, or Vista, you can get all the protection you need free from AOL. Sign up for a free e-mail account with AOL (do whatever you want with it), and McAfee SecurityCenter is yours free. It might surprise some people - it did me - to know that McAfee is currently the only vendor who holds ICSA certification in all three categories of traditional endpoint security: antivirus, antispyware, and PC firewall. This free suite includes those same products, and includes IDS, IP tracing (to help track down a hacker who tries to probe you), and script scanning. One of the biggest problems on the web right now is JavaScript iFrames, which contain a toolkit full of various malware samples. The criminals' idea is, if one attack fails, another might succeed. McAfee lets you block the whole thing by alerting you that a script is trying to run on your system, and letting you terminate it right there. Below is a screenshot to show you an example of what this looks like. Notice that TrendProtect is green. Many legitimate sites have been infected with iFrames, as phishers frantically respond to the success of web rating tools.