Makers of small web browsers want their programs to be given more prominence on Microsoft's browser choice screen.
Six software firms have complained to the EU saying many do not realise their programs were on offer.
To see all 12 web browsers, users must scroll to the right when viewing Microsoft's ballot screen.
The choice is being offered as part of a settlement of an anti-trust case brought against Microsoft by the European Commission.
"The final choice screen design leaves the vast majority of users unaware that there are more than five browsers to choose from," the six firms said in their petition.
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