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.

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.

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!)

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.

Good article on aicpa.org.

Source: http://www.freebyte.com/antivirus/.

svchost32.exe (NOT svchost.exe)
bingoo.exe (rare). 

If either process is running, select it and click “End Process”.

6. Look in C:\WINDOWS\System32\ (on Windows XP systems) or C:\WINNT\System32\ (on Windows 2000 systems) for:

2pac.txt
Bingo.exe
svchost32.exe

Delete all of these files that you find.

7. In the Registry Editor (“Start” -> “Run” type “regedit” and click “OK”) search (Ctrl+F) for “svchost32.exe” (without quotes).  Delete ALL instances of it.  There are usually around 10 instances of it.  Once you find one, hit the delete key and click “yes”.  To search for the next instance, hit the “F3” key.  Keep searching until no other instances are found (it will say “Finished searching through the registry”.
Repeat step 7 searching for “msgmr.exe”
8. Connect the machine to the network.
9. Run Windows Updates and apply all updates available.  (Explorer 6 update can be omitted to save time).
10. Ensure that you have the latest version of Symantec Antivirus program and the latest definitions.
11. Remove from network and run a full system scan with Symantec Antivirus.
12. Reboot the Computer.
13. Logon and check for the following files:

C:\funny_pic.scr
C:\see_this!!.scr
C:\my_photo2005.scr
C:\WINDOWS\System32\2pac.txt (XP)          C:\WINNT\System32\2pac.txt (2000)
C:\WINDOWS\System32\bingoo.exe (XP)           C:\WINNT\System32\bingoo.exe (2000)
C:\WINDOWS\System32\svchost32.exe (XP)          C:\WINNT\System32\svchost32.exe (2000)

If  any of them DO exist, go back to step 3 and repeat.  If these files do NOT exist, reconnect the machine to the network.
14. Check for and install any other Windows Updates that may be available.
15. Watch for Mytob symptoms.  If present, disconnect computer from network and go to step 3.
16. Enable System Restore (XP).

 Source: http://www.humboldt.edu/~cnrsitc/mytob

1. Beware of Toolbars et al.
2. FOSS instead of Free.
3. A Firewall and a good Antivirus are your Shields.
4. Beware of malicious sites.
5. Firefox instead of IE.
6. Pop-ups are bad.