If you see the HP BIOS error 500 or 501 on starting your system and you cannot boot into Windows, the solutions explained here will help you.

I have an old HP ProBook 6450b, based on an early first gen i5 M450. I've had Windows 10 32-bit on it for several years, and I'm now upgrading the RAM from 2x2GB to 2x4GB, 8GB Total, PC3-10600 ...

What gen? HP website should still have it for download. And the bios is by motherboard family so a search for the bios family should reveal similar models with bios updates that can be used.

The BIOS password is stored in a non-volatile memory, which can be cleared. Doing so clears the password too, allowing you to reset it. Here are several methods that you can use to recover your BIOS ...