Question : Problem: Corsair memory upgrade

I have tried upgrading my memory on my Asus P5L-MX board
and recieved slow results. This is what i have and did:

OS = Windows Vista Home Premium
MB = Asus P5L-MX  rated Max. 4GB DDR2 667 memory
CPU = Overclocked Pentium D 820 [2.8GHz] OC 3.15GHz
Original Memory = Corsair 2GB DDR2 667 # CM2X1024-5400C4  [ CL 4-4-4-12 ]

I tried using G.Skill  4GB DDR2 667 # F2-5300CL4D-4GBPQ [ CL 4-4-4-12 ]
upon switching, the system slowed down extremely.

I looked at these Corsair, but do not want to buy them if they will not work.
Corsair DDR2 800 #TWIN2X4096-6400C5 [ CL 5-5-5-18 ]
Does anyone know if these would speed up the system?

The system used to run very fast, but lately has been running very slow [ before any upgrade ]
This is why i thought that if i increased my GB in memory, it would help the system.
I have also done the following before upgrading:
defragmented
disk cleanup [ under hard drive properties ]
removed all files in the recycle bin
ran norton 360 2.0 system clean-up, optimize disks, etc. + did a full system scan for viruses, etc.

anyone know if i should upgrade the corsair memory to the 4GB?
or is there something that i am missing that i should do to speed up my system?
The Pentium D has always performed fast for me so i do not want to replace that unless needed?
Could it be some of the programs?
Norton 360 v2.0
Itunes
Standard Windows Programs
[ all games are played online with cable internet connection ]

Last note:
This is happening to 2 computers with the exact same build and programs.


thanks

Answer : Problem: Corsair memory upgrade

First, try to determine if it's a software or hardware problem. Run msconfig and disable all non MS services and all startup items. If performance is then normal sequence back startup programs/services until you find the one(s) causing problems. Start with Norton 360.

Download, install, update, and run Malware Bytes AntiMalware to ensure you don't have a malware problem.  http://www.malwarebytes.org/mbam.php