Richard Durkin, who is in his 40s and comes from Aberdeen, was awarded £8000 after the Supreme Court ruled a bank had breached a "duty of care", causing him to be blacklisted.
Mr Durkin said he was glad his "consumer victory" would help the "greater good", but that he had run up legal bills of about £250,000.
Mr Durkin bought the laptop at a PC World store in Aberdeen in 1998 and signed a credit agreement with HFC Bank for about £1500, the Supreme Court 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.
But HFC said he had to maintain 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Mr Durkin took legal action, arguing he had validly rescinded the credit agreement.
Five justices at Supreme Court in London ruled in his favour yesterday, saying he was entitled to rescind the credit agreement and had done so validly. They said HFC had breached a duty of care and awarded Mr Durkin £8000 damages.
Mr Durkin had taken his fight to the Supreme Court - the highest in the UK - after battles in Scottish courts.
"I've got mixed feelings, I suppose," said Mr Durkin, an offshore surveyor, after the ruling. "I'm glad I've helped the greater good with a consumer victory."
But he added: "I've got myself into a lot of debt, basically.
"There's a lot expenses still to be distributed; hopefully I'll get something back, I've put about £250,000 into it."
Mr Durkin's solicitor, Frances McCartney, said: "This decision has far-reaching implications for the financial industry and indeed the consumer. Credit providers will need to establish they made reasonable inquiry about the validity of a debt prior to referring (or even threatening to refer) an individual to a credit reference agency. It is their obligation. Failure to do so will lead to a potential claim for compensation if the debt is not established.
"Given the number of years that have elapsed since the Consumer Credit Act 1974, there are likely to be many thousands of people affected by this decision."
Mr Durkin had initially claimed £250,000 damages. He said HFC's actions meant he was unable to put down a deposit on a house. A judge had awarded him about £100,00 for loss as a result, but an appeal court overturned the decision. Mr Durkin yesterday failed to persuade the Supreme Court to reinstate the property loss damages award.