Registration for Window 7 Upgradation...

I have purchased Dell Laptop Studio 1555 on dated 19/10/2009. I am trying to register with https://win7.dell.com for upgradation window vista to window 7. After filling all the entries, it asks for service tag no. I entered it exactly same as given on the back of laptop. When it verifies it, it displays error msg. "The System is not in our database .......................................". I am trying for a long for so many times but could not succeded. kindly help me. I also tried to contact to customer care but problem could not be resolved.

Regards,