What should I do to protect my computer against viruses?

Virus

Install a licensed antivirus program such as Symantec, Norton, McAfee etc. and always keep it updated. An antivirus program that has not been updated is helpless against newly devised viruses.
Keep your operating system updated. You can update your Windows operating system from Settings -> Windows Update.
Instead of Microsoft Outlook or Outlook Express, prefer to use Mozilla Thunderbird or any of the webmail services provided by METU-Mail since these are more reliable against viruses that seep through gaps in Microsoft Outlook ve Outlook Express.
Unless necessary do not turn on file sharing. If it is a must, set password requirement on and select read-only file sharing.
Unless needed, do not set up server type operating systems such as Microsoft Windows Server etc.
Do not operate IIS (Internet Information Services) as a web server that have not been updated.
Set your computer to be booted only from the hard disk. Always back up your crucial files/information. Do not run macros on the office programs that you do not know.
For not to be affected of viruses which infect through Microsoft Office, use alternative Office programs such as OpenOffice.org.
Do not run the executable files which have been sent by e-mail as attachment unless you verify it from the sender.
Use anti-spyware to protect your computer against spyware which aims to collect your personal information.