The Tablet PC, a computer designed to function more like a portable writing tablet, faces an uncertain future, as Microsoft Corp. rolls out its next generation operating system for small computers, an analyst firm said Tuesday.

By 2009, Tablet PC sales are expected to reach $5.9 billion from $1.2 billion last year, In-Stat said. Beyond that time period, however, the future becomes cloudier, because of Microsoft''s scheduled launch in 2007 of the Ultra Mobile operating system for computers the size of a paperback books.

"It''s unclear how that will mesh with the Tablet PC," In-Stat analyst Brian O''Rourke said. "To me, (the ultra-portable OS) indicates a lack of confidence in the future of the Tablet PC."

Since its introduction in 2002, the Tablet PC has had mixed success overall. While it has sold well in industries with mobile workforces, such as health care, real estate and insurance, and in companies with large sales forces, the devices haven''t caught on in the general corporate market.

The latter would include middle and upper management who would want a computer to take with them from meeting to meeting. For them, the Tablet PC has proven too expensive and its screen size too small, O''Rourke said.

Over the next few years, however, sales are expected to improve, because vendors, such as Toshiba, Acer and Gateway, are starting to ship Tablet PCs with screens as large as 14 inches. In addition, the average selling price is expected to drop this year below $2,000.

With spending on information technology increasing, corporations may be willing to take a second look at lower-priced Tablet PCs, O''Rourke said.

Long term, however, sales could suffer, depending on how Microsoft positions the Ultra Mobile 2007 in the market, and whether devices with the OS compete with Tablet PCs.

"(With the Tablet PC) we''ll see short-term increases in shipments and adoption, but long-term uncertainty," O''Rourke said.