Just curious, when's the last time you used an XP computer with just 512MB RAM?
Have you ever run XP on the system before? Is the HDD blank or is there an OS currently installed? If there is an OS installed, what is it? Did you test the memory with memtest86 to confirm it's not defective & that the full 512MB is detected? Do you have a graphics card or integrated graphics? If integrated, how much memory is allocated to graphics? Is this a custom built PC? If so, please list the make/model of the motherboard along with the rest of the specs. If it's an OEM system (Dell, HP, Gateway, etc), please list the make/model.
Are you using a pirated copy of XP?