Displayed memory capacity is wrong

OS is WinXP SP2:
System came with 256 Mb of ram. (1 card)
The BIOS recognizes all of it.
Windows only recognizes 192 Mb.
I also get a Stop error (BSOD) which I assume is
related to the above.

0X0000008E (0XC000001D, 0XBF80E4A0, 0XF0068C88, 0X000000)
Win32k.sys – address BF80E4A0 at BF80
The event log is:
Source: System error, Category 102, Event ID: 1003
error code: 1000008E, parameter 1: c000001d
parameter 2: bf80e4a0, parameter 3: f0068c88
parameter 4: 00000000

Any help would be greatly appreciated – Thank you!