Hi Riptorn, Spybot is a great freeware anti spyware tool. I m using it and it works great. The only bug are:
1. One of its process Tea-Timer.exe takes more memory and thus can make other apps run slow(sometimes).
2. It integrates itself into IE and may make your browser slow.
3. For every system changes and registry changes, it gives annoying pop ups and asks for permission(Allow or Deny).

But its worth installing if you are concerned about the system security as it provides tight security.

Alternative: Malware Bytes Antimalware.

Hope that helps,
Dinesh.