Slow computer

I have been through the process a bunch of times, I know the drill and I check my threads often so the first to help I would greatly appreciate.

I had AVG 2012 on here that was expired but it was still working and catching threats. I ran a scan yesterday and it removed 2-3 rootkits. I had no issues up till that point. Just trying to optimize performance. I did however shut my PC down the other day and I got a box saying that RUNDLL32.EXE needed to be shut down and I clicked ok, shut down and when I restarted I got a DOS message stating that there was an invalid system disk. Fixed that by resetting bios.

Today I downloaded trial of WinZip and a new 30 day trial of AVG 2013 and now my PC is slow. I have a Logitech optical mouse (cordless) and it starts slowing down all the time, I click on things and nothing happens. I did a cleanup to the best of my memory, I havent done alot of stuff on XP in years. Forgot all the tricks. I went into MSCONFIG and shut down some start up programs and am trying to figure out how to shut down more things in task manager (non system related). I have a RUNDLL32.exe program in there running and not sure if its legit or if its a virus. I know that there are virus's that name itself as that.

So, the question is... from this point, what would be the best programs to run to see if there is something still on here slowing me down?

please post links to updated scanners.

thanks