My computer has a GIGABYTE GA-880GA-UD3H vers 2 motherboard, with a 3.40 GHz AMD Phenom II X4 965

BIOS: Award Software International, Inc. F4 07/28/2010

I’ve recently had to reformat my hard DRIVE, and reinstall Windows XP Professional SP3.

I have also replaced all the memory modules with 4 modules of 4GB each, totaling 16GB.

However, when I check Control Panel/ System Properties/General, it states 3.25GB of RAM.

When I run BELARC Advisor program, the following is reported for memory:

3326 Megabytes Usable Installed Memory

Slot 'A0' has 4096 MB
Slot 'A1' has 4096 MB
Slot 'A2' has 4096 MB
Slot 'A3' has 4096 MB
I continually get warnings of ‘high memory usage’ detected by Internet Explorer.

How do I get the computer to recognize and utilize all of the installed memory?
Install a 64 bit OPERATING sytem. Windows 7 64 bit Only support More Ram
But you have an option. if you use win 7 32 bit you need to patch to use total of you Ram
I'm not sure in win xp working. The Windows 7 patch to use more RAM is not recommended. As suggest by JJ3000, if your system supports 64 bit then you should install a 64 bit operating system to use more than 4 GB ram.