Microsoft's chief executive vowed to "kill Google" in an expletive-laden tirade against the firm, according to US court documents filed by Google.
The claim was made in a sworn statement by Mark Lucovsky, a former Microsoft employee who quit for Google in 2004.
Microsoft chief Steve Ballmer has denied the claims, saying they are a "gross exaggeration of what actually took place".
The statement is the latest salvo in a bitter legal battle between the firms.
In his sworn statement, Mr Lucovsky - a key Windows architect - alleged that Mr Ballmer hurled a chair across the room when he informed him he was moving to Google, before launching into an abusive tirade against Google's chief executive Eric Schmidt.
However, Mr Ballmer has dismissed the claims.
"Mark's decision to leave was disappointing and I urged him strongly to change his mind. But his characterisation of that meeting is not accurate," he said in a statement.
The row between the two firms was triggered when one of Microsoft's vice presidents, Dr Kai-Fu Lee, was hired by Google to set up a research centre in China.
Microsoft claimed the move was a violation of a one-year non-compete clause in his contract and began legal action against the search engine giant.
However, Google has retaliated by claiming that Microsoft's action is a form of intimidation designed to eliminate the threat of a fast-growing rival.
The group has been moving further into the software arena - most recently with the launch of Talk, a service which lets e-mail account holders talk to each other via a PC, microphone and speakers.
The system is a direct threat to online voice and instant messaging service providers such as Skype, Microsoft and Yahoo.
Microsoft won the first round of the increasingly bitter battle between the two firms in July, when a King County Superior Court judge issued a temporary order barring Mr Lee from carrying out the duties he had been hired to do for Google.
The two sides will face each other in court again on Tuesday when Microsoft will ask a court to extend that order until the matter comes to trial in January.