Windows – Slow Computer after RAM Upgrade

I have an older Acer laptop (2009) which only had 3GB RAM. I decided to upgrade it to 8GB RAM (2x4GB, same speed like the old one).

I also formatted my hard drive and installed a fresh copy of Windows 7 x64.

During the installation, I noticed that the laptop was extremely slow (took about 3 hours to install the OS). After Windows finished installing, the computer took almost 20 minutes to boot and it was basically unusable after it booted.

Considering this is a fresh OS install, I thought the new RAM could be the only issue (both the BIOS and the OS detected the full 8GB).

I put the old 3GB RAM in (1GB + 2GB sticks) and it was fast. At this point, I thought one of the new RAM sticks must be bad.

I took out the 3GB (1GB + 2GB) and put one of the new 4GB sticks. The laptop was fast. I tried the same with the other new 4GB stick and it was still fast.

I then thought that perhaps one of the slots are bad, so I tried each slot individually with only one stick and it was still fast.

Then I tried the following combinations:

• 4GB new + 1GB old = fast
• 4GB new = fast (with either of the two sticks in either of the two slots)
• 2GB old + 1GB old = fast
• 4GB new + 2GB old = slow
• 4GB new + 4GB new = slow

I have no logical explanation for this. It seems that for up to 5GB, it's fast. If I add more than 5GB, it becomes EXTREMELY slow (5 minutes to boot).

I tested the new RAM (2x4GB) in another laptop just to make sure and it's working great.

The only processes running on the laptop are the ones that run on a fresh Windows 7 installation.

I checked task manager when the computer is slow and I noticed that the CPU usage fluctuates between 30% and 100% even when laptop is idle. There isn't one single process that is eating up the CPU, but rather a few different Windows processes and the usage keeps going up and down without me doing anything on the laptop.

Any thoughts on this are appreciated. Thanks in advance!