I would recommend SUPERAntiSpyware, it's a very good all-round utility with a high-detection rate. The free version will allow you to perform a system scan. Be careful for false positives though since it's a powerful scanner.
Link: http://www.superantispyware.com/
In my opinion, Spybot S&D is long dead and causes more problems than its worth nowadays. Spyware Doctor is ok but it's too bloated for my liking. MalwareBytes is another good tool but I don't think it does as good a job as SUPERAntiSpyware.
Also check that you don't have too much stuff running in the background. If you're anti-virus solution is scanning and you're not aware of it, this could cause high CPU usage for example. Does the computer feel slow at all? Also if you use the Task Manager and check under the Processes tab it will tell you which processes are using your CPU and at what percentage.
If you notice any processes that are suspiciously high post back and we'll go from there. Good luck