About half of corporate PCs aren't equipped to run all the features of Windows Vista, and companies should gradually deploy the upcoming operating system on new computers, rather than take the more costly alternative of upgrading older ones, Gartner said last week.

Desktops or notebooks with less than half of their useful life left when Microsoft ships Vista shouldn't 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, according to Gartner. Microsoft is promising the operating system in January, and assuming it doesn't 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 they're ready to deploy it, the useful life left on 2006 PCs would be about 17% on laptops and 37.5% on notebooks.

Vista's requirements include a graphics card that supports the 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. These features won't be important for many companies, but others will be, such as better window stability, smoother screen drawing, and interface scaling.

37.5% The useful life likely to be left on 2006 notebooks when Vista comes out.

PCs also will need at least 1 Gbyte of RAM and an additional 512 Mbytes if companies plan to use PC virtualization during the migration to run an older operating system and Vista simultaneously, Gartner said. Upgrading RAM on a PC costs from $100 to $200 per machine.

"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."

In terms of upgrades, Vista is comparable to Windows 2000. Windows XP, which fell between the two, was considered a less radical change.

It's too soon to tell if this move will be a more costly hassle than the one to Windows 2000. "If the applications are more compatible with Vista than 2000, then it might be less costly," says Mike Silver, a Gartner analyst. "Migration techniques have gotten better, but most organizations have not automated their migration process."