My computer is running slow but has a good processor and plenty of RAM

Dell vistro 64-Bit Operating System

Windows 7 

4 GB RAM i7 

2 Hardrives

Computer was running extremely slow I tried adding some extra RAM but the PC wouldn't pick it up. I checked the processes and there was no unknown applications running so here are the steps I took to resolve this issue. 

I removed the 2 extra sticks of RAM in case they were corrupt since it seems that the PC was not able to identify them. 

I did a DELL system test and it did report some errors on the secondary hard drive. I ran a check disk and all seemed to check out ok. However I still went ahead and removed the second hard drive and everything seemed to go back to normal. 

I also did a disk defrag and a disk clean up, when I did a disk clean up there was a huge amount of system memory error logs and were taking up 230GB of space on the computer I cleaned that up and the computer when completely back to normal.