Microsoft is providing free phone support for Windows Vista Service Pack (SP) 1 users — whether they get their SP1 from Microsoft or via a PC maker or other retailer — through March 2009.

There’s got to be a catch, right?

Surprisingly, the caveats are relatively minor. The help is specifically for SP1 installation and compatibility issues. And the offer stands through March 18, 2009.

According to M3 Sweatt, the Chief of Staff for Windows Core Operating System Division (COSD), the offer of free SP1 help is not unprecedented. Microsoft has provided free support for previous Windows service pack installations.

Sweatt blogged on March 24 about the procedure those needing help should follow:

“If you call in for support, listen carefully to the options on the line as you wait and navigate for support. When you speak with an operator, be sure to tell the customer support agent that you would like support related to the installation of Service Pack one (or ‘SP1′) of Windows Vista. I’ve also seen the wait time drop from 30 minutes for individual chat support to less than 10 just in the last few minutes.”

The toll-free call-in number in the U.S. for Vista SP1 help is (866)-234-6020.

Microsoft made Vista SP1 available to a broader set of customers last week, by making the collection of Vista fixes and features available on Windows Update, the Microsoft Download Center and via retailers like Microsoft is set to push Vista SP1 to users via its Automatic Update mechanism in mid-April.