Windows Vista took longer to arrive than hoped, and it might not have everything that was once planned, but Windows chief Jim Allchin maintains "It's a big deal."

And, more than five years after the debut of Windows XP, Vista is finally ready--at least, for businesses willing to buy at least five copies of the operating system. Those companies can get it starting Thursday, while consumers and those looking to get a Vista-equipped PC will have to wait until January.

For those who have been tuning out all the Vista chatter for the last few months, here's a primer on the new Windows. The update has security improvements, some snazzy new graphics and new desktop-searching abilities, among other features.

So is Vista really here?
After months of issuing community preview versions, beta versions and release candidate versions, Microsoft has finally declared Vista soup.

Large businesses can start getting Vista through volume-licensing contracts as of Thursday, while CompUSA is selling licenses to smaller businesses that purchase at least five copies of Vista. However, consumers and those looking to get new PCs with Vista installed will have to wait until the mainstream launch in January.

What if I buy a new PC now? Will it still run Vista?
Microsoft is offering an "Express Upgrade" program that runs through early next year. It offers those who buy an XP machine now a free or discounted copy of Vista, once it starts shipping to consumers.

There's still the question of how Vista-ready the PC is. Microsoft is using two logos to help consumers get a sense of that. Some machines are billed as "Windows Vista Capable." A PC with that logo will be able to run Vista, but that sticker does not guarantee the computer will have enough graphics horsepower and other components needed to run all of the operating system's new features. Those who want to guarantee that should look for the shiny "Vista Premium Ready" logo.

So what's in this Vista thing?
Vista--which used to be called Longhorn--has evolved quite a bit since Microsoft first demonstrated an early version in 2003. The company has dropped plans to include its all-new WinFS file system. It has also changed the way it's implementing a new Web services architecture, known as Indigo, and a new graphics engine, dubbed Avalon.

Among the key features of Vista as it currently stands are: security enhancements, a new searching mechanism, lots of new laptop features, parental controls and better home networking. There will also be visual changes, thanks to Avalon, ranging from shiny translucent windows to icons that are tiny representations of a document itself.

On the business side, Microsoft said Vista will be easier for companies to deploy on multiple PCs and that it will save costs by reducing the number of times computers will have to be rebooted.

Vista includes antispyware tools, Internet Explorer 7, an update to its Web browser, as well as Windows Media Player 11. It also has Windows Calendar, a new systemwide tool designed to do for datebook information what Outlook Express does for e-mail in Windows XP.

Is that all?
No. Among the other features Microsoft has publicly confirmed are: broad IPv6 support, improved clientside caching of data stored on a server, whole-volume encryption, a revamped synchronization engine, the ability to support laptops with an auxiliary display, automatic hard drive optimization and a secure boot-up process that helps prevent someone from gaining access to your data if your PC is lost or stolen.

Will my PC run Vista?
That depends on how recently you bought it and just how much Vista you want. To get the basics, like the new search abilities and improved security, you'll need a PC with 512MB of memory, an 800MHz processor and a 20GB hard drive with at least 15GB of free space. But to see Vista in all its glory, particularly its new Aero graphics, you'll really need a relatively modern video card with around 128MB of dedicated graphics memory or, for a system with shared systems and graphics memory, you'll need 1GB of memory.