I was wondering, however, if I could reinstall Search & Destroy, and SuperAntiSpyware. Or do they conflict with the other things I have got installed? And also, would you recommend installing another firewall from this page viewtopic.php?p=557962#p557962 ? I know Avast has one, and I do have the Windows one, but you are the professional!
Yes, you can reinstall the Spybot Search & Destroy as well as SuperAntiSpyware, both of them should run fine together.
Actually, your computer must have a different protection programs, but only one of each species - one antivirus, one firewall, one antimalware searcher, etc.
You have Avast as antivirus and it is a good choice.
If you add Malwarebytes' Anti-Malware scanner, chose between WinPatrol and Spybot Search & Destror, i wouldn't run both together as they basically do the same job.
Regarding your doubts about firewall, I can say that previously during Windows XP era we did not recommend to use the built-in Windows XP firewall, because it did not block outgoing connections. This means that any malware on your computer is free to "phone home" for more instructions. Simply put, the Windows XP internal firewall was a very basic.
The Windows 7 firewall is two way so there is no real need to install a third party firewall!
Please remember, if you decide to install a third party firewall, only one firewall at the same time should be active!.
Please don't hesitate to ask any additional questions.