If you have an antivirus program installed and it's up-to-date, you probably don't have a virus infection. There's always the possibility, however. Here are some ways that viruses can sneak by:

• If you don't keep your virus definitions up-to-date, a new virus can infect your computer that your software doesn't know about yet.
• If you disable your virus checker (for example, to install new software) and forget to enable it again, it doesn't do you any good.
• If your virus checker becomes disabled for some reason and you don't notice it, you can become infected.

You Probably Have a Virus If . . .

First, let's look at reasons to strongly suspect virus infection on an end-user's computer. These are all symptoms that are rarely caused by anything except a virus:

• You received an email with an odd attachment and opened it, with unexpected results such as odd dialog boxes or a sudden drop in system performance.
• There's a double extension on an attachment that you recently received and opened, such as .jpg.vbs.

It's much easier to spot double-extension files if the display of extensions for known file types in Microsoft Windows is turned on. To do that select Tools > Folder Options, and uncheck the Hide extensions for known file types checkbox on the View tab.

• Your antivirus program is disabled for no apparent reason (perhaps with an X through its icon in the notification area) and cannot be enabled. It may report an error condition.
• An antivirus program will not install on the computer or appears to install but then will not run, but other programs will install.
• Odd dialog boxes or messages appear on-screen.
• Many files are missing, especially many files of a common type. For example, some viruses have a side effect of deleting all the files of certain graphics types.
• Others let you know that they have recently received odd emails from you containing random attached files or a virus.
• The computer starts doing things on its own that it would normally never do, like the mouse pointer moving by itself, windows opening or closing, programs running, or the CD tray opening and closing by itself. This is a symptom of someone actually using a backdoor to operate your computer, rather than a symptom of the existence of the backdoor itself.
• When you look in the Users section of the Control Panel, you notice new users with full security permissions that you know you did not create, or you notice inappropriate permissions assigned to existing users. Again, this is a symptom of backdoor hacking, more than a symptom of a virus infection per se.
• Odd icons appear on the desktop that you did not place there, and you have not installed any new applications lately that could have placed them there.
• Strange sounds or music plays from the speakers for no apparent reason.
• File sizes or date and time stamps have changed on files that you know you did not alter.
• A program that you used successfully recently has disappeared, and you know you did not uninstall it.

There are some types of viruses called retroviruses that specialize in disabling your antivirus software, so monitor your antivirus software frequently to make sure it's on-the-job.