Mallwarebytes is rated as one of the best and used by malware removal experts as a first line of removal before going into the more technical systems, some of which are based on major improvements to "Hijack this". The technical programs such as OTL, Combofix, and several others are not to be used by the untrained.
However Spybot was once a good program but is no longer rated and regarded as outdated, instead there is "Superantispyware as a complimentary program to Malwarebytes.

Remember that these programs are stand alone on demand scanners and removers and not active in the same way an antivirus or firewall software is.

Recently being recommended is the Microsoft Security Essentials as an antivirus software and is improving continuously with each update.

Avira is also another good free antivirus too.

For a stand alone firewall, please stay away from the all in security suites, they become resource hoggers but Comodo firewall on its own is still a useful firewall, though from Windows Vista upwards, the Windows own firewall is at the least adequate.

Malwarebytes, although linked to earlier in the thread is here

http://www.malwarebytes.org/

once the Malwarebytes main page opens, hover mouse pointer over "Products" and click on "Malwarebytes antimalware free" to get the free version.

For Superantispyware download from here=

http://www.superantispyware.com/

Microsoft Security Essentials from here=

http://windows.microsoft.com/en-GB/windows/products/security-essentials

Avira antivirus from here=

http://www.avira.com/en/avira-free-antivirus

Comodo Firewall only from here=

http://personalfirewall.comodo.com/

The free firewall download is on the right of the page.

You can find me as Plastic Nev, on Free PC Help, Computer Help Forums, and also 5Star support. More of a computer tech than a musician these days.