Signs of viruses: Are you infected?

After you open and run an infected program or attachment on your computer, you might not realize that you've introduced a virus until you notice something isn't quite right.

Here are a few primary indicators that your computer might be infected:
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Your computer runs more slowly than normal
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Your computer stops responding or locks up often
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Your computer crashes and restarts every few minutes
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Your computer restarts on its own and then fails to run normally
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Applications on your computer don't work correctly
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Disks or disk drives are inaccessible
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You can't print correctly
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You see unusual error messages
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You see distorted menus and dialog boxes

These are common signs of infection—but they might also indicate hardware or software problems that have nothing to do with a virus. Unless you run the Microsoft Malicious Software Removal Tool and install industry-standard, up-to-date antivirus software on your computer, there is no way to be certain if your computer is infected with a virus or not. If you don't have current antivirus software installed or if you're interested in installing a different brand, you can try Windows Live OneCare free for 90 days or visit our Windnows Marketplace for software from other companies.
Tip

Tip: Beware of messages warning you that you sent e-mail that contained a virus. This can indicate that the virus has listed your e-mail address as the sender of tainted e-mail. This does not necessarily mean you have a virus. Some viruses have the ability to forge e-mail addresses.


Source by: http://www.microsoft.com/protect/computer/viruses/indicators.mspx