Don Young, analyst at UBS Warburg, warned that Microsoft had struck new contracts with a handful of large enterprises, mostly based in Europe.
Under current Microsoft licensing arrangements, high volume users often sign three-year enterprise agreements, with fees paid annually on a per-user basis. After three years users are allowed to continue using the version of the software installed under a "perpetual use" clause and only pay more if they want to upgrade.
However, Young said Microsoft had recently struck fixed-term deals that did not include the "perpetual use" clause. This means users will have to continue paying Microsoft once the contract ends.
The change would protect Microsoft's revenue stream from slowing PC sales, said Young. A Microsoft spokeswoman refused to comment on the analyst's specific allegations but admitted the company was looking at changing licences.
"There will be some announcements coming out soon, but we have no comment at the moment," she said.