The browser kept trying to contact a domain he was unfamiliar with. Mainwaring, who runs a website offering a subscription service for horse-racing statistics, thought he might have become infected with malicious software.
Fearing his customers' data could be compromised, Mainwaring called BT, his broadband service provider. BT concluded he must have had a virus.
"I started to turn white," Mainwaring said.
But the problem persisted, even after he wiped his hard discs clean and bought a new PC. Mainwaring started to investigate and found that the domain the browser was trying to contact belonged to 121Media, a company now called Phorm.
Phorm created a targeted advertising system called Webwise, which three UK ISPs so far have agreed to trial. Webwise monitors a person's web browsing in order to serve relevant ads.
Since advertisers will pay a premium to reach customers that fit a certain profile, ISPs employing target ad systems would get a cut of the revenue.
Phorm says the system does not retain personal information, but UK privacy activists question whether the system violates wiretapping regulations. A similar debate is ongoing in the US concerning the company NebuAd, which is marketing a similar system.
BT along with Carphone Warehouse and Virgin Media plan to trial Webwise. But BT has further aggravated privacy activists after it secretly tested Webwise on 18,000 subscribers over a two-week period in September and October 2006.
Most people didn't notice the test. But Mainwaring did, along with 15 to 20 other people who complained through customer service channels.
Mainwaring was one of several people who passed out leaflets about Webwise yesterday outside the London venue for BT's annual meeting. The calm protest was intended to raise awareness of the technology with BT shareholders, as well as press the UK government to investigate whether BT violated wiretapping regulations.
The UK has been fuzzy over which government agency has jurisdiction over investigating the trial, said Alexander Hanff, a law student who has written a dissertation about how BT may have violated the law. Hanff said it appears the City of London police have the right to investigate if they choose, and he planned to provide them with his dissertation, as well as other materials regarding the secret BT trial.
Phorm also appears to be drawing more political attention. A peer in the House of Lords, Sue Miller attended the protest. Miller said she planned on Thursday to press Home Office Undersecretary of State for Security and Counterterrorism Alan West for more clarification on ISPs and interception regulations.
Miller said BT officials visited her on Monday and assured her the Webwise system complies with the law.
Nonetheless, some ISPs, both in the US and UK, are steering clear of targeted advertising systems for fear of losing customers.
"The real reason I don't touch Phorm is because it's illegal," said Jason Clifford, who runs an ISP called the UK Free Software Network. "I think it it's going to be a very poor return on investment."