You should definitely have an ant-spyware program that provides real-time protection. Most of the ones I've seen you have to pay for. You don't have to pay for Windows Defender.
For on-demand scanners in addition to Ad-Aware SE and Spybot, I use SuperAntiSpyware Free, AVG Anti-Spyware 7.5 (used to be Edwido), and A2 Free.
I also have SpywareBlaster which prevents a lot of junk from getting on the computer.
Orange Blossom