Product validation and equipment

[Jack]

I bought a second-hand computer a year or two ago and I find that it
won't accept my valid Windows product keys for Vista and XP (I want to
put these on my spare HDD just for software-experimental purposes). Is
this something the manufacturer has done to the Bios? Am I not legally
entitled to have these OSs activated? What can I do? (I read Windows'
online advice on how to activate by phone, but the options that the
passage claimed would appear when I typed into the 'Start' search box
what I was asked to type in, did not appear.)
Indeed there seem to be further issues with software validation -
particularly with regard to limited-time trial versions - generally.
With thanks for any help.