For decades, Microsoft has subsisted on the milk of its two cash cows: Windows and Office. The company's occasional ventures into hardware generally haven't ended well.

But the new Surface Pro tablet, which goes on sale Saturday, seemed to have more going for it than any Microsoft hardware since the Xbox.

Everybody knows what a tablet is, right? It's a black touch-screen slab, like an iPad or an Android tablet. It doesn't run real Windows or Mac software -- it runs much simpler apps. It's not a real computer.

But with the Surface Pro ($900 for the 64-gigabyte model, $1,000 for a 128-gig machine), Microsoft asks: Why not?

The Surface Pro looks like a tablet. It can work like a tablet. You can hold it in one hand and draw on it with the other. It even comes with a plastic stylus that works beautifully.

But inside, the Pro is a full-blown Windows PC, with the same Intel chip that powers many high-end laptops, and even two fans to keep it cool (they're silent). As a result, the Pro can run any of the 4 million Windows programs, like iTunes, Photoshop, Quicken and, of course, Word, Excel and PowerPoint.

Now, when I wrote a first-look post on my blog last month, I was surprised by the reader reactions. Over and over, they posted the same argument:

"For that money, I could buy a very nice lightweight laptop with a dedicated keyboard and much more storage. Why should I buy Surface Pro when I can have more for less?"

Why? Because the Surface Pro does things most laptops can't do. Like it weighs 2 pounds, with touch screen. Or work in portrait orientation, like a clipboard. Or remain comfortable in one hand as you make medical rounds, take inventory or sketch a portrait. Or stay in a bag as it goes through airport security (the TSA says tablets are OK to stay in).

The Surface Pro is fast, flexible and astonishingly compact for what it does; that much is unassailable. But in practice, there are some disappointments and confusions.

The Surface Pro runs Windows 8, which is two operating systems in one. You get a tablet operating system, and the regular Windows desktop underneath it. You wind up with two Web browsers, two control panels, two Mail programs, two completely different looks.

This machine has two modes -- tablet and laptop -- so its two operating systems each serve a purpose.