memory - Windows only sees 768 MB RAM out of 4 GB

I have a Windows XP machine with 4 GB of memory. As expected (and discussed in a dozen or so other questions on this site) the OS could only see 3 and a bit... until yesterday. As of yesterday, the "System Properties" window shows only 768 MB RAM.

dxdiag shows the same thing... actually, it reports 766 MB. On the other hand, the BIOS still shows 4096 MB, and msinfo32 reports "Total Physical Memory: 4,096 MB."

I've never seen anything like this before. Where could the other 3.25 GB of my memory possibly have gone? Is some faulty driver mistakenly mapping most of my memory to some piece of hardware?

UPDATE: Quick summary of the situation so far: The BIOS shows 4 GB. MemTest86+ shows 4 GB, and all of its tests pass. The msinfo32 utility shows that 4 GB are installed in the computer. However, both Windows XP "System Information" and SliTaz Linux report only 768 MB of memory. MemTest86+ seems to conclusively rule out a hardware problem, but two operating systems seeing the same thing rules out a software problem! So, where do I look next?

UPDATE 2: I tried resetting the CMOS back to factory defaults: no luck. I tried taking out one of the sticks (I have 2x2 GB) and booting: the computer only saw 768 MB. So I took that stick out and put the other stick in: 768 MB. I took them both out and put them in the second set of slots: 768 MB. Finally, in the name of trying everything, I installed the two modules in the "wrong" arrangement, i.e. not in a matched pair: Still 768 MB. No matter what arrangement of memory I have, the computer only sees the first 768 MB.

Answer

Open the Device Manager and under the View menu, choose "Resources by connection". Expand the Memory node and see if you can figure out what, if anything, is occupying the memory. If you're only seeing 768MB, there's probably an entry starting at 3000 0000 named "PCI bus". Expand that node, and see if there's anything actually at 3000 0000. (RAM is "System board".) You can then try disabling that hardware, perhaps installing an alternate (video, network, drive controller, etc.)