Windows 7: 64bit 4gb installed 3.12 usable?

The BIOS must support the memory remapping feature. The memory remapping feature allows for the segment of system memory that was previously overwritten by the Peripheral Component Interconnect (PCI) configuration space to be remapped above the 4 GB address line. This feature must be enabled in the BIOS configuration utility on the computer. View your computer product documentation for instructions that explain how to enable this feature. Many consumer-oriented computers may not support the memory remapping feature. No standard terminology is used in documentation or in BIOS configuration utilities for this feature. Therefore, you may have to read the descriptions of the various BIOS configuration settings that are available to determine whether any of the settings enable the memory remapping feature.

i had the same problem. In the morning i had 4gb of ram installed and working by evening only 3 working (4 showing). Now i had installed updates earlier today so i thought that might have something to do with it. i tried a few solutions on here nothing worked for me. i checked bios, it was fine. when I restarted my pc msn started so i went into msconfig to switch it off in startup(type msconfig into run) out of curiosity i clicked on boot-> advanced options and for some reason the box maximum memory was checked. so i unchecked it and restarted and now i have 4gbs again. I think one of the security updates must've have ticked the box. But try this anyway, i hope it works for you