Under this program, to debut tomorrow, anyone who buys a PC from a participating manufacturer or retailer that is fitted with a Windows Vista OS will receive upgrades to at little or no cost. Windows 7 is set to debut in 14 languages, including English, on October 22. The upgrade program runs until January 31.

But there is a catch: there is a limit of 25 PCs per individual under the program. A single company counts as an individual, a Microsoft representative added on Thursday.

"Specifically designated PCs that are pre-installed with Windows Vista Home Premium, Windows Vista Business, or Windows Vista Ultimate may qualify for an upgrade to the equivalent Windows 7 product," a Microsoft representative said. "Similarly, in some markets, retail-packaged Windows Vista software products (sold separately from PCs) may also qualify for an upgrade to the equivalent Windows 7 product."

Manufacturers, or OEMS, and retailers may charge a nominal stocking or shipping fee pertaining to the arrangement.