I'm kind of doubtful the installation of Windows is what is causing the problem. The OP very likely doesn't have the memory speed set higher than JEDEC default of 2133Mhz and may have even installed the modules incorrectly so it's running single channel mode.
First we should check your memory settings by downloading and installing CPU-Z. Run it and switch to the Memory tab and check where it says Channel #. If you have 2 or 4 modules installed correctly, it will say dual for dual channel mode. If you only have one module or if you installed 2 or more modules incorrectly, it will say single. Still on the memory tab, what does it say for DRAM Frequency? Moving to the SPD tab, click the drop down box and select the slots until you find a memory module. Type out what the part number is for one of the modules. Once we know what memory kit you have from the part number, we can figure out what speed your memory should be set.