HP Pavilion XE748 does not recognize 256MB memory mods

[sjwalsh]

I purchased two 256MB PC100 CL3 memory modules from Viking to upgrade my HP pavilion XE748. The manufacturer stated that this memory module is fully compliant with the XE748. But, when I remove the original 64MB module and plug in either one or both of the new 256MB mods the computer beeps a 1-3-3-1 code and just sits there. I checked the HP website and it read that the 1-3-3-1 code and gave its description as Autosize DRAM, and gave a Recommended Action of "Troubleshooting the system memory" - helpful, NOT.
Anyway, I have cleaned the sockets, reseated the new modules dozens of times, downloaded the updated BIOS for XP (the original PC came loaded with Windows Me), but the only way the PC boots is with the 64MB module in it. I can put one of the 256MB module along side the 64MB module and the PC boots, but on checking System Resources the 256MB module is not recognized as being there - justs reads that the slot is empty.
Well, after about 7 hours of trying I am at my wits end - I even tried emailing the manufacturer, but all I hear from them is the Zen sound of one hand clapping.
Steve

[embj]

Well, those MF's at HP don't know what they are talking about...You need PC133. I tried to install PC100 in one of my friend's HP XE748 and it wouldn't work. So I went out and bought 2 256mb sticks of PC133 and it worked. So you need PC133 and not PC100. You should never trust HP. They are idiots. Also, they said that I couldn't install a 512mb stick of RAM into one of my HP's...haha...it worked.