If you've let your guard down--or even if you haven't--it can be hard to tell if your PC is infected. Here's what to do if you suspect the worst. Heard this one before? You must run antivirus software and keep it up to date or else your PC will get infected, you'll lose all your data, and you'll incur the wrath of every e-mail buddy you unknowingly infect because of your carelessness. You know they're right. Yet for one reason or another, you're not running antivirus software, or you are but it's not up to date. Maybe you turned off your virus scanner because it conflicted with another program. Maybe you got tired of upgrading after you bought Norton Antivirus 2001, 2002, and 2003. Or maybe your annual subscription of virus definitions recently expired, and you've put off renewing. It happens. It's nothing to be ashamed of. But chances are, either you're infected right now, as we speak, or you will be very soon. For a few days in