I think I wore out my old HD when my system was failing- thought it was a virus at first so reinstalled XP and tried to save files - then that didn't work well so I installed some various Linux op systems to see what I liked compared to XP-- formatted and reinstalled over and over XP/linux etc- no matter what I tried sys kept failing and having issues til it totally died.

so that was when I gave up and got the new MB and HD.
Started form square one since old HD was more or less toast.

I don't know how to get around the password issue

How old is your system?

[Question is, he said, a new MB+CPU may not match the original BIOS+Windows XP software in which case I still could not gain access to my old HDD password protected data.]

Do you have a name brand pc?? - HP, Dell, gateway etc -- those do have proprietary OS I believe most anyway. HP for sure.

I know HP - probably most manufacturers - have a chat, email, or support section you can get information from them.
I ordered replacement OS from HP {for another machine I upgraded for kid} for only $18 - but turned out the MB was bad on it also we got a new non- HP MB, HD - so now it is just a generic and we can use our own copy of XP on it.

if you add a new generic {not the name brand} MB your proprietary OS will not work.