"PC manufacturer's max memory may differ" HELP!

About the only thing that would change mobo settings would be if HP had a BIOS upgrade that would change this. I'm guessing that HP put on this restrictions so they could sell a similar model for more money. Quite honestly, given how old that machine is, I wouldn't put any money in it and would be very careful updating the BIOS. Once a machine gets so old, if it runs, you're best off leaving it as is. The old if it aint' broke, don't fix it rule applies