Wrong memory count

I have a precision 220 with 512MB RAM. BIOS sees all 512 but WIN XP will only use 124MB. Systems Properties screen displays “124MB RAM”. If I use a different HDD with WIN XP on it – all 512 is available which leads me to believe MEM & MOBO are OK. Bios has been upgraded to A13(most recent). There is too much to be lost by formatting this HDD.
Anyone know how to correct the amount of mem Win XP sees?