My current triple threat for treating computer slowness

     "...all free today"

* CCleaner (also known as Crap Cleaner), developed by Piriform, is a utility program used to clean potentially unwanted files (including temporary internet files, where malicious programs and code tend to reside) and invalid Windows Registry entries from a computer.^[6] 

Get rid of the "junk in the trunk" first. Run the cleaner and the don't forget to "fix" your Registry errors.

http://en.wikipedia.org/wiki/

* Malwarebytes' Anti-Malware (MBAM) is an application for computers running under the Microsoft Windows operating system that finds and removes malware.^[4]

Search and destroy the bad guys with a malware scanner. You may have to run the scan several times. Be prepared for some restarts of your computer along the way...

http://en.wikipedia.org/wiki/Malwarebytes%27_Anti-Malware

* Microsoft Security Essentials (MSE) is an antivirus software (AV) product that provides protection against different types of malware such as computer viruses, spyware, rootkits and Trojan horses. It runs onWindows XP, Windows Vista and Windows 7, but not on Windows 8, which has a built-in AV component. 

Run a first class anti-virus product for FREE. This latest Microsoft offering gets some hight praise, and is becoming and industry standard.

http://en.wikipedia.org/wiki/Microsoft_Security_Essentials

What else can I do? (Handle with care) 

* MSCONFIG - find out what's in your "Startup" - check and uncheck items to enable or disable these programs during startup.

* HiJackThis - http://en.wikipedia.org/wiki/Hijack_this

Bottom line, there is a bunch of things you can do before spending any extra money.