How to tell, what to do if computer is infected

Computer-virus infections don't cause your machine to crash anymore. Nowadays, the criminals behind the infections usually want your computer operating in top form so you don't know something's wrong. That way, they can log your keystrokes and steal any passwords or credit-card numbers you enter at Web sites, or they can link your infected computer with others to send out spam. Here are some signs your computer is infected, tapped to serve as part of "botnet" armies run by criminals:

• You experience new, prolonged slowdowns. This can be a sign that a malicious program is running in the background.
• You continually get pop-up ads that you can't make go away. This is a sure sign you have "adware," and possibly more, on your machine.
• You're being directed to sites you didn't intend to visit, or your search results are coming back funky. This is another sign that hackers have gotten to your machine.

So what do you do?

• Having anti-virus software here is hugely helpful. For one, it can identify known malicious programs and disable them. If the virus that has infected your machine isn't detected, many anti-virus vendors offer a service in which they can remotely take over your computer and delete the malware for a fee.
• Some anti-virus vendors also offer free, online virus-scanning services.
• You may have to reinstall your operating system if your computer is still experiencing problems. It's a good idea even if you believe you've cleaned up the mess because malware can still be hidden on your machine. You will need to back up your files before you do this.

How do I know what information has been taken?

• It's very hard to tell what's been taken. Not every infection steals your data. Some just serve unwanted ads. Others poison your search result or steer you to Web sites you don't want to see. Others log your every keystroke. The anti-virus vendors have extensive databases about what the known infections do and don't do. Comparing the results from your virus scans to those entries will give you a good idea about what criminals may have snatched up.

http://tech.yahoo.com/news/ap/20090315/ap_on_hi_te/tec_inside_a_botnet_checklist