Avast is free for one year, and is fast and light at 27 mb of RAM. We use this for all the old beaters that roll in for service, it works great on XP.
Malwarebytes is a good general purpose spyware cleanup tool. We don’t recommend the resident component, it uses too much memory.
It is free security software and it works well on modern PCs, we use it on all our new builds. Not reommended for old beaters.
Spybot is a good general purpose spyware cleanup tool.
OpenDNS is a great tool for reconfiguring your router to block vast chunks of bad internet. Check it out.