Activation code don't work anymore

I am using a legal copy of Window xp, I.ve used this copy twice before over the past 18 months. Due to a virus infection I opt to reformat and reinstall, but when it came time to enter the activaction code, i get an error that the code is not valid, i've typed this code twice before with success,but now it is not reconized, my question, is there a way to compare the code on the disk with the one supplied on the case. I've carfully retyped this code many times and it is still not excepted. I don't think this is part of the infection after the reformat.



thanks