In a conference call with financial analysts, Ballmer said lower selling prices, limited corporate sales and software piracy may result in lower revenues than expected, ARN News reports.

“I'm really excited about how enthusiastic everybody is about Vista,” said Ballmer. “But people have to understand that some of the revenue forecasts I've seen out there for Windows Vista in fiscal year 2008 are overly aggressive.”

Ballmer added that he expected Vista to see greater sales growth in emerging markets such as in China, India and Brazil, but that this growth will be on a smaller base than in developed countries, and in high-piracy markets.

And while Microsoft sold a lot of Vista upgrades to business clients, Ballmer said that many have already been accounted for in previously signed contracts, so forecasts of additional corporate sales “may be a little more bullish” than is warranted.

Ballmer did not provide specific sales numbers.