I have a DUAL XEON 2.8 with 4gb ATP PC1700 ram. The 4gb shows up in boot,
bios, and windowsXP system information, but NOT via my computer or task
manager.
I have tried the /PAE and /3GB(which limits it to 3gb) boot.ini switches to
no avail.
Please advise.
-JB

It must depend on your mainboard brand only. For instance, Intel explains it
in its 'Server Products' FAQ section:

Not All memory Is Available After Installing 4 GB of System Memory



After installing 4 GB (gigabytes) of system memory, the operating
system is only reporting 3.5 GB of system memory available. This is
expected, and by design.

Memory between 4 GB and 4 GB minus 512 MB (megabytes) will not be
accessible for use by the operating system and may be lost to the user,
because this area is reserved for BIOS, APIC configuration space, PCI
adapter interface, and virtual video memory space. This means that if 4 GB
of memory is installed, 3.5 GB of this memory is usable.

The chipset should allow the remapping of unused memory above the 4 GB
address, but this memory may not be accessible to an operating system that
has a 4 GB memory limit.