Windows says that my credit card number is invalid - it's not?

When I try to make a payment on a web site all goes well until I click on 'submit', then a Windows box pops up saying that my credit card number is invalid. It's not and until a few days ago I had no problem making payments. My other card, which is also OK gets the same error message.
I know that it is a fault in the Microsoft validation program but I don't know how to correct it. I tried using System Restore, going back to a date when things worked but this hasn't helped. I've also tried installing IE7 and Firefox but I always get the same message. Microsoft technical help and knowledge base are no help at all.
I am running Window XP Home with Service pack 2 and most updates and IE6 with service pack 1.