This is hardware. Is the machine still under warranty? Have you checked the FAQ on the hp.com site?
No warrenty and yes, I have gone through the site.
ANyway, I have now cleaned the contacts of the RAM and also the socket to no avail. The first time after boot, it didn't give the error code, but after the memory check, colors just started flashing on the screen. After a rebooting, it went back to square one: black screen (monitor saying no valid input) and the computer hanging.
I am reseting the CMOS now and hopefully I can try a couple of other things... but this is where I am now.
Btw, the RAM is a 512 DDR PC-2100 with CL 2 I believe.