Microsoft warned people in the U.S. of a phone scam offering free computer help Thursday.

Callers have been posing as computer engineers and calling people at home, telling them that their home computers are vulnerable to a security threat. The callers claim to represent established companies and offer free computer help. They tricked some people into giving the caller remote access to their computer, downloading malicious software and providing their credit card information.

The company commissioned a survey by Dynamic Marketing in April that reached 7,000 people in the U.S., Canada, U.K. and Ireland. Fifteen percent of respondents said they had been called, and the number rose to 26 percent in Ireland.

Of the people who received the call, 22 percent ended up getting scammed and losing money. The average amount lost in the U.S. was $875. The average cost of repairing affected computers was $4,800 in the U.S.

While the scam targeted English language countries, Microsoft believes it will spread to other countries as well.

If you believe you've been affected, Microsoft gave this advice: Change the passwords on your computer, your main email account and all your banking and financial accounts. Use a safety scan software to detect any malicious software. Alert your banks and credit card companies.

To avoid getting scammed, Microsoft offered this advice:
Be suspicious of callers who offer free software help. Don't provide financial information to unsolicited callers. Don't go to a website or download software based on an unsolicited call. Take the caller's information down and report to law enforcement. Use security software ad keep it up to date. (Microsoft recommends its own free product, Microsoft Security Essentials, which you can download here.) Change your passwords regularly and use a strong password. Make sure you have a firewall installed.