If you put the Ubuntu CD in, does it bring up the installation screen?
If so, there's not too much wrong with the motherboard. Could also be a bad hard drive, or hard drive partition setup. If you can get the Ubuntu installation screen, run a recover. If the installation did not complete, the installed system is almost certainly not bootable. There is a lot of vital stuff set up right at the end of the onstall, like the bott loader and partition.
You don't say what model HP/Compaq you are using, but many of the require ACPI, APIC and LAPIC or a combination disabled on install in order to get the install to work.