Computers tend to bog down over time. Some simple maintenance every 2-4 weeks (depending on computer use and programs installed) will only take 10-30 minutes, can be performed at any knowledge level and will keep your computer running smoothly. With this guide, and two downloads, you will see a noticeable improvement. This is fairly basic and something all computer users should do.

1. Remove all programs you no longer use or knew about.
2. Do not use Multiple Antivirus Applications or Software Firewalls.
3. Cleaning your hard drive of unneeded files.
4. Remove invalid registry entries.
5. Startup items: remove all unused programs.
6. Defragment your hard drive.

Remember: slow PC is not always caused by spyware or viruses. It could just be due to what you run!

 Full instruction and more good advices can be found here.

Good comparison of antiviruses. You can check it before purchase.

If you are running Windows 2000 or Windows XP try this. Do not use on older operating systems.
Follow the directions carefully.

Click here to download the trial version of Ewido Security Suite:
http://www.ewido.net/en/download/

Install Ewido.
During the installation, under “Additional Options” uncheck “Install background guard” and “Install scan via context menu”.
Start ewido.
Look for the update tab on the left on the screen, and update the program to the latest definitions
DO NOT run a scan yet.

Restart your computer into Safe Mode now.
(Start tapping the F8 key at Startup, before the Windows logo screen).
Perform the following steps in Safe Mode:

Start Ewido:
Click on scanner
Do a COMPLETE SYSTEM SCAN.
During the scan you will see a prompt asking you to clean files, click OK. You can check the bottom box to “perform the same action on all infections”

This scan make take some time, so be patient.
It may be helpful to turn off your system restore before you scan. Most infections return upon reboot because they are stored in the restore files.
  

Just a little fix - use safe mode for all operations.

(via Yahoo.Answers!)

Short, but highly-illustrated tutorial.

An expert’s look on the problem. Detailed explanation.

When running the freshly minted copy of Windows, the old infested copy can either be visible to it or not. If it is visible, then data files can be copied from it to the new Windows instance as needed. And, you might use anti-virus and anti-Spyware software running in the new clean copy of Windows to remove the malware from the old copy. If you think you’ve cleaned out the old copy of Windows, then you may want to boot it to run your applications. If so, be sure to hide the new copy of Windows from the old copy - just in case there is still an infection.

The most effective spyware programs display no symptoms, so the computer user is unaware dirty tricks are being secretly perpetrated while the machine is running. Less-sophisticated spyware, however, causes various symptoms. The most common are persistent pop-ups that appear even when you aren’t surfing the Internet or are unrelated to the content you are browsing. Other symptoms include sluggish computer performance, unauthorized changes to your Internet browser’s default home page, the sudden appearance of new browser toolbars and even random crashes. In addition, dial-up Internet users may suddenly discover unidentified 900-number telephone toll charges caused by spyware programs known as dialers.

Good article on aicpa.org.

Looking like Made-For-Adsense page, but tutorials are ok. Check it out.