I'm a Systems Engineer and everyone I know is constantly asking me to clear viruses and spyware from their computers. And when diagnosing issues with games, people will routinely suggest you need to make sure you have a clean any possible infections on your PC. This is my generic advice for cleaning your PC, and preventing infections. Everything I mention below is a completely free product. I'm not selling anything, just giving advice.

The long and short is that I recommend Firefox + Adblock Plus, a HOST file, Microsoft Security Essentials and Spybot Search and Destroy. Below, I'm going to get a little more in-depth.

NEVER, NEVER use Internet Explorer. Let me repeat. NEVER, NEVER use Internet Explorer, unless you want viruses and spyware. Run Firefox with Adblock Plus and/or Noscript. Google Chrome is also a pretty safe browser, but it doesn't block ads and scripts the way Adblock Plus and Noscript does in Firefox.

If you ever think you have an infection, do the following:

Download Process Explorer from here:

http://technet.microsoft.com/en-us/sysinternals/bb896653.aspx

That will show you every process running on your computer. You can Google up the process names to find out what they are, and kill the ones you don't want running. This might kill the virus so you can clean it properly. The normal Task Manager might tell you that you don't have permission to kill a process, and some viruses stop you from loading the task manager. But you can always load Process Explorer. The worst thing you can do killing processes is crash Windows, forcing you to reboot.

Download and install a HOSTS file. This will block tons of domains. You're computer simply can't talk to them. The domains on this list are known to spread viruses or serve up ads. Even by itself, a HOSTS file will block a whole bunch of ads.

http://www.mvps.org/winhelp2002/hosts.htm

Download Spybot Search and Destroy from here:

http://www.safer-networking.org/en/

When you install it, don't install TeaTimer or the Realtime protection. The anti-virus we're going to choose below will take care of that. Download the latest updates from within the app, then run a spyware scan. Also run the immunize feature when you're finished.

There are several free anti-virus products. The best (hands down) is Microsoft Security Essentials. It is completely free, updates automatically, doesn't eat up too much memory (and won't nag you about upgrading to a pay version).

http://www.microsoft.com/security_essentials/

If for some reason you can't run Microsoft Security Essentials (you're running Windows 2000) there is AVG Free, but it will harass you to upgrade to a pay version. It eats up more memory, and doesn't detect viruses as well as Security Essentials.

http://free.avg.com/

If you just want to run a scan without installing any software, there is Housecall, which is also free:

http://housecall.trendmicro.com/

Check to see if the virus created a "service" that are runs in the background. It is also a good idea to disable services you don't need to make your computer start up faster. Some services, like Remote Registry, are legit services from Windows, but allow people remotely to change your registry (Windows settings). If you have a home computer, there is zero reason to run this service. It is just a security issue waiting to be exploited. Here is a guide to help you check Windows services.

http://www.blackviper.com/WinVista/servicecfg.htm

Also check the "Startup" folder in your Start Menu. There are also items hidden in your registry that startup in the background. Disabling unneeded apps from loading in the background will also speed up your PC, even if they aren't spyware apps. For instance, many people have Java Updaters, Google updaters, Acrobat Speed-Launch, and various driver apps that load in the background that you have zero need for. Most of the driver apps aren't the drivers themselves, but rather apps that let you access the driver from your system tray. If you're unsure if you need the app, use Process Explorer to kill the app. If you're fine, then you can likely remove it from startup.

http://support.microsoft.com/kb/270035

The HOSTS file, Spybot's immunize feature, and anti-virus should protect you from getting new infections in the future. I again highly recommend surfing the web with either Firefox or Chrome. If you use Firefox, install Adblock Plus. That alone does wonders to make the web safer.