Microsoft's pledge to allow easier access to rival browsers in Windows by the middle of May, ended a long antitrust dispute with the European Union.
The company has started to send a choice screen, where consumers can easily click on rival browsers, to almost 200 million old and new computers.
According to web statistics firm Statcounter, Internet Explorer's share of all Web surfing has in March dropped in France by 2.5 percentage points from February, in Britain by 1 percentage point and in Italy by 1.3 points.
Norway's Opera Software, the fourth largest browser firm, has seen downloads more than double in Europe from normal levels due to the choice screen, with downloads in Italy, Spain and Poland more than tripling.
Also No 2 browser firm, Mozilla, says it has seen strong growth. "We have seen significant growth in the number of new Firefox users as a result of the Ballot Choice screen. We expect these numbers to increase as the Ballot Choice screen fully rolls out across all countries," said a spokeswoman for Mozilla.
Smaller Web browsers have urged the European Union to push Microsoft to provide them more visibility in its browser choice website.
At first sight, Microsoft's browser Choice Screen (www.browserchoice.eu) shows its own Internet Explorer, Firefox, Opera, Apple Inc's Safari and Google Inc's Chrome. It is not immediately obvious remaining choices are available by scrolling to the right of the Web page.
"To date, new downloads of Flock originating from the browser choice screen have only contributed marginally to growth in overall downloads. This is also the case for the other browsers not on the main screen," Flock said in a statement.
Microsoft has said the screen complies with the EU's decision. The five largest browsers show directly on the Choice Screen, but smaller vendors say there is nothing to show consumers that more options are available.
"We hope that the changes recommended in our urgent petition to the European Commission are implemented so that all the browsers that have been placed to the right of the main screen will have a reasonable chance of being found and considered by European consumers," Flock said.