installed memory 6 gb (2.67gb usable) help

First check when your PC starts what amount of RAM does it detect. I had a similar problem recently and one of my motherboards couldn't detect my second stick of RAM.

If you go to your bios during starting your PC you can often see in the bios screen how much RAM your bios detected.

Upgrading the bios can ruin your computer's BIOS if you do it wrong so please follow the instructions for the specific model of your motherboard.

What motherboard do you have? Check the make and model number of your motherboard and Google the Make and model and add support to the search string. It should show you the official support page for your motherboard. There you can find drivers and tool and BIOS. If you are lucky you can run a windows BIOS upgrade tool that will do everything for you. If you are unlucky, you have to download the BIOS and put it on a memory stick or a DVD and go into the "easy" BIOS flash utility.

It is important to note that since you have 6 gigs of RAM you probably have a 4gb stick and a 2gb stick, and it is possible that your motherboard will not support that configuration even after a BIOS upgrade. Perhaps the best solution is to replace your 2gb stick with a 4gb one that is the same model as the one you already have. You can also try sticking the second RAM that isnt detected into another slot, maybe it will pick it up.

Good luck.