If your computer has slowed significantly in its operation and you are suddenly getting pop-up ads constantly, then your computer is probably infected by a virus. Slowed performance is a prime indicator that you have a problem. The first thing to do is remain calm and develop a strategy to deal with the situation. If you do not have anti-virus software loaded on your computer, then fixing your PC will be difficult, but not impossible.

First Steps

Unplug your computer from the Internet. More than likely your virus is sending information to some other computer on the Internet, and the best way to salvage your data is to stop it from being transferred to another PC. If you can still use your PC, then try to back up your personal data on to a CD, DVD or USB drive. The virus may infect your backup information as well, but you at least have to try. A virus will either start taking effect immediately or it will start to work its way into your directories; but wait for a full re-boot before it starts to destroy files. In order to stop it, you need to have the latest anti-virus software on your PC. If you do not have current anti-virus software, then you will have to get it and load it on to your computer.

Anti-Virus Software

The problem with a virus is that it may be affecting your ability to access the Internet. So while the Internet is full of valuable virus attacking tools for you to use, if you cannot get there, then they cannot help you. If you have updated anti-virus software, then run it immediately. Your software may be able to isolate the virus or eliminate it. If you do not have anti-virus software, purchase it and load it on to your computer. Do not re-boot your computer until you have anti-virus software loaded on it. If the virus is set to attack on a re-boot, then even newly loaded anti-virus software can handle it.

Safe Mode

Larger viruses can become extremely intrusive, and sometimes they can do so very quickly. If your anti-virus software does not work, then re-boot in safe mode to give your anti-virus software one more chance. Booting up in safe mode means opening up only the very basic operating system. Many of the video, audio and communications drivers that viruses feed from will remain closed. This gives your anti-virus a chance to attack the virus while it is essentially standing still. You may have to re-boot in safe mode several times to remove a persistent virus, but your chances of success in safe mode are much higher than in a normal boot sequence. To go into safe mode, simply hit the F8 button on your keyboard just as your computer is starting to boot up, then follow the on-screen prompts to load in safe mode.

Reformatting

If all else fails, then you will need to reformat your hard drive to get rid of your virus. This means that everything on your hard drive will be lost, including your operating system and all applications. If you have hard copy backup of your operating system and applications, then you can just reload them after you have reformatted your hard drive. To reformat your hard drive, put your operating system boot disk into your CD drive and start up your computer. Choose the option to boot from your CD and follow the instructions.

Source:

Microsoft: How do I Remove a Computer Virus?

SpamLaws.com: I have a Virus. Now What?

"PC World" Magazine: Caught a Virus?

More Information:

BleepingComputer.com: How to Remove a Trojan, Virus, Worm or other Malware

Safe Mode

Reformatting a Hard Drive