Windows only sees 768 MB RAM out of 4 GB

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.)

"Some ideas of what to try:

Clear the CMOS
Unplug the power and remove the RAM for 30 minutes, reseat it well.
If you have access to another identical computer, try exchanging RAM.
Take the computer to a repairman that will have the tools and spare parts required to run all the needed experiments."

"I came across this post yesterday when I got a similar memory issue with a Windows 7 laptop sporting 3 GB RAM, but only reporting 766 MB available.

Having read through various posts I decided to physically change the available RAM by simply removing one of the SODIMMS. So my 3 GB laptop was temporarily 1 GB (removing the 2 GB module). I suspect as mentioned before, re-seating or swapping over good RAM would not make a difference.

And voila! 1 GB RAM was suddenly seen. I then removed the 1 GB module replacing the 2 GB module - and 2 GB was seen and usable. Then replaced the 1 GB and the 2 GB and all 3 GB was available, not the 766 MB or whatever it was.

I also adjusted the video RAM in the BIOS and although this had the effect of incrementally changing the available RAM, it did not fix the problem for me.

The original issue appears to have been caused by (of all things) the Windows validation/activation. Following a dead hard disk and no recovery software, I used a retail version of Windows 7 64-bit to re-install (knowing the EULA was incorrect). I then re-activated Windows with the correct EULA (from the base of the laptop). Although Windows was happy and re-activated OK, it was following this (and apparently it is fairly common) that available memory issues arose.

It appears Windows reserves too much memory (incorrectly) for other hardware use. I also uninstalled some hardware devices along the way, but this made no difference either."

"You could test the system using a BartPE build of Windows. The easiest software to acquire for this is Hirens10.1

I would recommend Hiren's as it is quick to load and deal with. During the boot just select mini windows; it should be the third option down. Once you are in mini windows you should be able to check to see if it is a fault of your current Windows installation or if it a memory or motherboard fault.

In my opinion no technician is complete without these tools, so you might as well get a copy."

"FIX FOR ME:

I had the same problem with an Acer Aspire laptop running Windows 7 showed 3 GB in Windows, but only 768 MB usable, but I thought of the previous answers and thought ""why would this start playing up overnight after months of trouble free use with no driver updates?"" So I removed the 1 gigabyte stick leaving the 2 gigabyte stick in and there was no POST.

I replaced the 2 gigabyte with the one stick I had removed and post fine. DA DA, faulty 2 gigabyte stick! I replaced the 2 gigabyte stick, and I now have 2.75 usable out of 3 gigabyte."