HTTPS sites fail.

Hello friends!
I do apologies if this has already been covered and fixed but for some odd reason I cannot access secure websites. I can browse the internet till my eyes turn square(er) Before posting I did check out the other suggested threads and their suggestions but to no avail.
I have 2 PC's hooked up to the same router, one being Vista, the other XP. It is only the XP that fails to be able to access the HTTPS sites which leads me to think that it is a PC problem rather than my router problem.
Also it is a newly formatted (or "destructible restore" I think HP put it, HD is partitioned) which I hoped would of solved the problem but apparently not, I do not have firewall software (unless you count the router firewall which would block the vista PC from going on the same sites if it were the problem)
So if there are any suggestions from you kind folk or if you need any other specific information give me a shout, would but most appreciated
Cheers muchly.