The Information Commissioner's Office (ICO) says it has received 524 complaints about unsolicited calls made by Amber Windows between May 2011 and April 2013.
And officials said an investigation found it was ringing people who had registered with the Telephone Preference Service (TPS), which is banned under the Privacy and Electronic Communications Regulations.
The Birmingham-based window company, which has a branch in Wolverhampton city centre's Cleveland Street, denies the allegation and says it will fight the fine. The ICO has also issued an enforcement notice against the firm, ordering it not to call subscribers who have previously said they do not want to receive marketing calls, as well as anyone who has not consented and those who have registered with the TPS for at least 28 days.
ICO director of operations Simon Entwisle said: "We wrote repeatedly to Amber Windows asking them to stop; they ignored us.
"Now they are facing a £50,000 penalty and a legally enforceable order to cease their unlawful practices. We will continue to target these companies that bombard households across the UK with nuisance calls and texts."
The firm has been trading for three decades and has six branches selling double glazing and panels for doors and conservatories, among other products.
Managing director Ben Roberts said: "We are saddened and shocked by the outrageous decision announced by the ICO. We dispute almost all of their findings and the amount of the fine is completely disproportionate.
"We shall be appealing this decision, as we always try our best to work within the law, which is constantly changing."