Microsoft's Internet Explorer browser was unsafe for 98 per cent of 2004, according to security firm ScanIT.
The researcher's controversial findings also confirm Mozilla browser to have been unsafe for a relatively short 15 per cent of the year. ScanIT's figures come from a survey of 195,000 Internet users.
Those using Firefox suffered the shortest "exposure period" when a patch for known vulnerabilities was not there.
Internet Explorer enjoyed only seven days without being vulnerable between October 12 and 19.
ScanIT CEO David Michaux said: "For 200 days in 2004 – that’s some 54 per cent of the time - there was a worm or virus exploiting one of those unpatched vulnerabilities."A detailed account of the findings is available here.