Could you have by accident installed 32bit windows? I know you said it's x64 but it sounds like you're pressing that 4gb limit (~3.25gb addressable)
I can confirm it is indeed x64 via the Control Panel's basic system information. I have the same distro installed on two other rigs, one of which is an Intel Bad Axe 2 seating 1 390x, 1 290x & 1 290 w/ 2 x 1 GB + 2 x 2GB where all 6GB are properly allocated and the other from vague recollection is an Asus P5b also on socket 775 with 2 x 2GB where only 3.25GB is usable as indicated in the motherboard's manual.
fo00ok
Could you humor me for a sec and run cpu-z to see what kind of processor you're running? Just a quick search on the pentium d and there are conflicting answers that some pentium d's are only 32 bit capable. (also the p5b should be able to handle 8gb of ram) You can install a 64 bit OS on a 32bit only processor. I've done it on accident, I put W7 x64 on my dell mini 9 and it worked for what I needed it to do. If your processor is 32bit that would answer the 2 problems, 3.25gb of available ram and not being able to run this 64bit miner.