dell 4600i Xp re-install fail

hello, i was re-installing my OS, (Xp) and i was removing sp2 but then had to leave the house for a good while so i decided to do it when i came back from home so instead of shutting it down i decided to do a cold boot and it shut down. As i came back to re-install Xp i turned on the power and watched my comp boot up, saw the dell log and then usally the Xp logo would follow after. Unfortunaly it didnt come up and im forced to press alt+ctrl+del to restart to the dell logo and only option im left with is the bios setting.

Now originally my computer specs are these:
cpu-3.06 p4
Ram-512mb
Os- XP

a couple months back i upgraded the ram to add another 512mb in order to have 1g i did not do this i paid a tech guy to install it for me

Anyways my comps been running slowly and i wanted to save myself some cash and reinstall the os myself, though when im in the bios the error i get is the memory has been increased and will not boot to windows xp and will only display a black screen. what im assuming is that the extra 512mb of Ram is conflicting with original bios settings and im concluding that i must remove it in order for it to boot up to xp again and then reinstall the additional 512mb in order to gain 1g. or is there some alternative problem leading my comp from not entering windows xp,
thnx for ure time and hopefully i can get to the bottom of this.

the bios post error i am gettings says "memory size is different" if that helps

by removing i meant uninstalling sp2, and my comp doesnt even get to windows it just goes pitch black, and when i read the bios log it gives me a "post error, memory size is different" msg

thnx so much g-paw that help im back up and running, thnx again man = )