Years ago, I read a tip which has worked well for me for numerous re-installations of XP: Use the telephone system!
When you operate the "validate" option, you are asked whether you want to validate over t'internet, by telephone or in some other way. Opt for telephone validation. This should give you a freephone 0800 telephone number to an automated Microshaft validation computer. By this time your copy of XP will have generated a validation code. Enter this when the telephone call is answered. It will invariably claim that your copy is illegal and tell you what to do. Unless it tells you to push the button on your screen to generate a new validation code, ignore it and push the button anyway. This will now request the serial number of your copy of XP from the label on your computer case. Enter this and a new authorisation code will be generated. Enter this via your telephone keypad and you should be told that your copy is valid and the required authorisation code will be given to you. (notice I said "should"!)
P.P.