Maybe all you need is a new hard drive??

Have someone install it, copy over the OS files- or did you ever make those back up/ restore cds?
they could be used to set up a fresh system on the new drive and keep all the old stuff on the old drive as your back up files.

I don't know if HP is still proprietary , meaning they have to use HP stuff /operating system to repair/rebuild...
unless you do a full rebuild motherboard and many internals.

a good tech guy should be able to estimate what you have, vs new, vs replacing a few things to help you figure out the best way to go from here.