OK. Step one is to make SURE you cleared the CMOS. The PC must be unplugged when you take out the battery and to be sure about this leave the battery out for a long time, half hour or so, or you can also leave the battery in, unplug the PC and short the Clear CMOS jumpers. If you are quite sure this was already done and you have checked your power connections carefully then we may need to assume the BIOS update failed. The BIOS is a chip on the motherboard and can sometimes be re-flashed/recovered/replaced, though replacement would likely be impractical. I would contact HP support and tell them you are pretty sure the BIOS update failed and see if they can offer you anything. They may have a simple fix for you.
If they cannot help then you could try to get a new motherboard, or a used one, that is an exact match for the one you have, maybe by buying one on Ebay. If you can find an exact make/model match then your Windows install will boot on it and may not even ask to be re-activated. If it asks to be re-activated you should have no problem doing so since this is a valid repair of the PC.