Make a written demand (sent by certified mail, Fed-Ex or messenger to prove receipt) on Sears for the Windows 7 program or for its value. You can also dispute the computer purchase with your credit card company, if you paid by credit card. Failing a satisfactory response to either of those, you can sue in Small Claims court.
How were you promised Windows 7? If your computer purchase papers have any mention of this deal to include Windows 7, include that in your demand, as well as any ads or online notices that either company has. If it was a sales rep, include those details.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.