Microsoft server 2003 standard edition r2 x64 bit product shows invalid product.

Dear all,

I am facing a problem, to register  Microsoft server 2003 standard edition r2 x64 bit. My product key is ok. But on time of registration this product it will not accept my key or on Microsoft product registration support it's show its a invalid product. 

Please help me to register this product. 

Thanks

Puneet