About half of corporate PCs are not equipped to run all the features of Windows Vista, and companies should plan to gradually deploy the upcoming operating system through new computers, rather than take the more expensive alternative of buying new hardware for older machines, a research firm said Monday.

Desktops or notebooks with less than 50 percent of their useful life left when Microsoft Corp. ships Vista, expected in January, should not be upgraded, since the cost would exceed replacing it with a new Vista-enabled machine at the end of the older computer's life cycle, Gartner Inc. said.

Assuming Microsoft does not suffer another delay, the cutoff point would be computers bought in 2006 or earlier. Gartner advises companies to replace notebooks every three years and desktops every four years. Given that most companies will take at least 18 months from the time Vista ships for planning and testing, by the time those organizations are ready to deploy the new OS, the useful life left on 2006 PCs would be about 17 percent on laptops and 37.5 percent on notebooks.

Among the major requirements of Vista, compared with Windows XP or 2000, is a graphics card that supports Vista's user interface and visual enhancements, which include translucent window frames and task bar, real-time thumbnail previews and task switching, enhanced transitional effects and animations. While these features within Aero won't be important for many companies, other improvements in the UI will, such as better window stability, smoother screen drawing and interface scaling.

In addition, computers will need at least 1GB of RAM to run Vista, and an additional 512MB if companies plan to use PC virtualization during the migration to run an older OS and Vista simultaneously, Gartner said. Just upgrading RAM on a PC costs from $100 to $200 per machine for many companies.

"Based on what we have seen so far, we believe that, for most large organizations, it will not be possible to fully justify the cost of a full forklift migration of all PCs," Gartner said in its research note. "This is partially because of the cost of most companies' manual migration process."