My advice comes from my personal experience and previous testing of many of the spyware removal programs out there.

First of all understanding the difference between Viruses & Spyware:
If you are currently running a very popular virus protection software like Norton or McAfee and believe that your software will do the 'complete' job then you are completely mistaken. We have run a complete virus check on a particular PC and have found nothing. After the scan has been completed we have immediately run a Spyware scan and have found hundreds of Spyware infections on the same PC. This confuses the majority of people due to the fact that both viruses and spyware appear to be the same to peoples misconception.

Viruses..
Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages. Even these benign viruses can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.

Spyware..
Spyware programs can collect various types of personal information, such as Internet surfing habit, sites that have visited, but can also interfere with user control of the computer in other ways, such as installing additional software, redirecting Web browser activity, accessing websites blindly that will cause more harmful viruses, or diverting advertising revenue to a third party. Spyware can even change computer settings, resulting in slow connection speeds, different home pages, and loss of Internet or other programs

From testing all the different software out there the following seem to be the most popular
- spyware doctor
- adware pro
- counterspy
- spyware sweeper
But the best by far is Spybot search and destroy by safer-networking and to add to this is that the software is completely free.

First of all run this program with the internet turned off and after a fresh reboot. When you do run this for the first time, don't be alarmed at the amount of infections that it finds and that your virus protection has completely missed.
After scanning which usually takes 20-30mins, spybot 'may' ask you to reboot and rescan. If this is the case then do so. The reason for this is because some of the spyware infections can not be removed at the time of that scan because they are currently running and need to be shut off to remove.

Once completed reboot again and you will find your PC will be running so much faster.