Motherboard 256mb problem

[ronlegant]

I have system with the following spec
Dell Optiplex 3mhz processor
1mb memory
XP Sp3
160mb drive
In the middle of use I got the dreaded bluse screen and couldn't get any further
I had to put in the systems disk and reload xp which it appeared to "relatively succesfully" but when I restart the system I get
"Alert: OS install mode enabled. Amount of memory limited to 256mb and F1 to continue and F2 setup
When I press F1 I get to windows but am unable to do very much inc unable to recognice added hardware
Windows Systems tell me I have only 256mb
I have checked the 1mb memory elsewhere and I know it works
Any ideas please