Have a look at
this for validation issues, their causes and resolution.
This bit may be helpful if you have been sold a (quality) pirated version:
Microsoft will send a complimentary copy of Windows XP to customers who have been sold a high-quality counterfeit version of Windows XP. You must submit proof of purchase, the counterfeit CD, and a counterfeit report that includes details about the purchase. To obtain more information about the Complimentary WGA Kit and to apply, visit the following Web site to try to validate the software, and then select See all options
A faster option may be to call the telephone number that should be shown when validation fails or this number: 0870 60 10 100
Good Luck!