Pavillion XL754 + 2 PNY 256MB RAM Chips = Boot Loop

I am trying to upgrade my HP Pavillion XL754 to the max 512MB of RAM. I initially bought a PNY PC100 256MB chip. I already had an Infineon 256MB chip installed. When I installed the new PNY chip and powered up, it tried to boot once, then came back to the blue HP screen, then popped into a black screen saying there was a problem loading the OS. After talking to tech support at PNY, they suggested I put 2 identical chips in. I bought another PNY 256MB PC100 chip with the same result. However, either chip can be put in either slot by itself and the computer boots just fine. BTW, I previously had the 256MB Infineon and a 64MB Infineon running with no probelms. Any thoughts?

I am running XP Professional.

Oh, another thing, I can get into the bios upon startup and the system actually recognizes the 512MB of RAM.

I did come across another post where the user ending up re-installing XP...I'd like to avoid that if possible

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=512931