I presume you have the Intel D915GAG? When posting questions it really is good practice to give all the information.
I fix tractors for a living and I really do question my customers first, that way I can narrow down the reason for their problem, but here goes.
A couple of things jump right out at me. If you carefully go through them, you will either fix the problem or find the reason for this occurring.
1. Are you sure that you have the memory sticks in the correct slots? Checking on this site I find that your board likes its memory in the first slot if using one stick; first and third if using two sticks; and first, second and third if using three sticks. I think you said your memory is identical, so from that, you are using two or more sticks, this should give you dual configuration, IF they are of the same type AND frequency.
2. Are the memory sticks actually supported by this board? Have you checked this? You can do so at this site. If the memory is not supported then the O.S. will try to use the C;/ drive as memory and use further system resouces doing so.
3. Is this a new install or not? The reason for asking is that malware will use resources, especially Trojans! You may use various software to remove these, but to ensure that you REALLY do have a clean system, I would go to Majorgeeks and take "Chaslang's advice for malware removal.
The latest bugs are very good at not revealing themselves to normal methods. His method is involved and very few people have the patience to properly carry this out. It will take the best part of a day to do. The results however will reward you with a snappy, quick computer, that is a joy to use.
Hope this helps you