I use Ad-Aware for spyware, Win Patrol for the windows system, Zone Alarm for the firewall, and Avast! for everything else. All are free versions. I also recommend CCleaner and Defraggler. I send the CCleaner folks $20 a year, because they always seem to have an update.
I suggest this order.
1. Ad Aware and run it.
2. Avast!, run it.
3. CCleaner, run the cleaner for both programs and windows. The run the registry cleaner. Save a copy of your registry to the desktop. Then tell it to fix all the problems.
4. Win Patrol, install it.
5. Defraggler to defragment your hard drive.
You should notice a tremendous improvement right away.