Richard Durkin bought the laptop at a PC World store in Aberdeen in 1998 and signed a credit agreement with lender HFC Bank for about £1,500, Supreme Court justices heard.
He returned the computer the next day because it did not have an internal modem, and asked for the credit agreement to be cancelled.
HFC said he had to keep making payments and after he refused the bank issued a default notice.
It told credit reference agencies he had defaulted and his name remained on a credit register for several years, justices heard.
Mr Durkin took legal action, arguing that he had "validly rescinded" the credit agreement.
The Supreme Court today ruled his favour, saying he was entitled to rescind the credit agreement and had done so "validly".
Five justices delivered their decision at a hearing in London.
Mr Durkin took his fight to the Supreme Court - the highest in the UK - after losing an appeal in a Scottish court.
The five justices made a unanimous ruling in Mr Durkin's favour.
They said HFC had breached a "duty of care" and said Mr Durkin was entitled to £8,000 damages.
"HFC treated him as being in default and intimated that default to credit reference agencies. Mr Durkin claimed damages for financial loss caused by the damage to his credit," said one justice, Lord Hodge, in a written ruling.
"The principal issue in this appeal is whether Mr Durkin was entitled to rescind the credit agreement."
Lord Hodge said he was "satisfied" that Mr Durkin had been entitled to do so.
"I would allow the appeal and declare that Mr Durkin was entitled to rescind and validly rescinded the credit agreement by giving notice to HFC," he added.
"Damages resulting from HFC's breach of its duty of care are confined to injury to Mr Durkin's credit in the sum of £8,000."