Microsoft ( MSFT), already locked in a regulatory battle with European regulators over past behavior, could open a new can of worms with its next-generation Vista operating system, according to a report. The European Union is weighing a probe of the U.S. software maker about the possibility Vista could violate antitrust provisions, according to the Wall Street Journal. No decision on a formal investigation has been made. Last week Vista's release was pushed back to 2007 by Microsoft to provide more time for testing. The concern about Vista centers on two functions: search and security, the Journal said. Regulators are concerned Microsoft will push its own programs for both purposes, possibly disadvantaging competitors like Google ( GOOG) and Symantec ( SYMC). Microsoft didn't comment on the story. The EU's concerns are similar to those raised in its previous investigations of Microsoft. Two years ago, the EU ordered the company to offer a version of Windows without its Media Player software, to share communications code with rivals and pay a record $613 million fine. Microsoft is appealing the ruling. The stock closed at $26.90 Tuesday, up 3% on the year.