This tip is more of a guideline than a step by step as viruses can take many forms.

One can identify viruses by researching the symptoms. Specific error messages are especially helpful in this endeavour.

Try going to Google.com and type the error message into the search box surrounded by quotes. You may find an article or newsgroup where the solution was found.

For the less fortunate, error messages may not be popping up and you are experiencing weird behaviour instead. Do a little bit of testing and see what behaviours you are able to reproduce consistently.

For example, opening your web browser causes your computer to freeze, and this happens every time you try it. Now go to Google.com and type in keywords that briefly describe the problem. For example: computer freezes open browser. Try to keep it to Just keywords so your search isn't cluttered.

An alternative is to check Symantec's antivirus site. They usually post info on the latest viruses going around and how to fix them: http://securityresponse.symantec.com/

Of course, the best way of combating viral infection is to be aware of how they are spread and to prevent infection before it happens. An extremely useful guide to this can be found here