The new operating systems will run on 64-bit Intel Xeon and Intel Pentium 4 processors, as well as AMD Opteron processors and AMD Athlon 64 processors. The new software costs the same as its 32-bit counterparts.

The biggest gain will be seen by businesses and PC users who are running 64-bit systems with more than 4 gigabytes of memory, which is the upper limit of 32-bit processors. Today, most PCs come with 256 megabytes to 512 megabytes of random access memory. XP x64 features full 32-bit application support. The bonus is that you can run two or more memory-intensive applications now without bogging down the system, assuming you have enough physical RAM.

Bill Gates, Microsoft's chairman and chief software architect, says the market is ready to make a rapid transition to 64-bit technology, which will bring substantial performance improvements.

Gates also said the combination of the new Windows system and more powerful processors would enhance security against spyware, fraud schemes known as "phishing" that use fake look a like websites that aim to steal sensitive data.

"The new x64 versions of Windows will take advantage of 64-bit architecture advances that help block some of the most destructive worms and exploits of recent years," Microsoft said.

Hank Levy, a University of Washington computer science professor who started researching operating system structures for 64-bit computers more than a decade ago, said he is happy to see Microsoft coming out with a 64-bit version of Windows that can be adopted widely. But he said it could have happened much sooner.

"I would have liked to have seen it in 1995," Levy said. "The dominance of Microsoft and Intel on the desktop, and the lack of an x86-compatible 64-bit CPU, has basically held back the widespread adoption of 64 bits.