Changing memory

I have a PC with an AMD 1.2gig CPU, and 768 meg of SDRAM on board.
I recently loaded Windows XP Pro on my machine, and then decided to remove a 256 meg memory module to put in another machine.
Now every time I try to start my machine XP shows an error on loading, and says that I should go back to startup and change the amount of memory shown.
However on startup the correct amount of 512 meg is shown.
The only way around the problem is to restore the extra module.
Any ideas, short of reloading XP?