Detailed info for phone validation is on this page: How to contact a Microsoft Product Activation Center by telephone, but essentially as Phil described:

  To start the Product Activation Wizard, follow these steps:
  
      Click Start.
      Click All Programs, and then click Accessories.
      Click System Tools.
      Click Activate Windows.
  
  In the Product Activation Wizard, follow these steps:
  
      Click Use the automated phone system to activate.
      Click the drop-down menu, select the location that is nearest to you, and then click Next.
      The Product Activation Center telephone number is displayed at the top of the window.
  

I used it once years ago after I changed a motherboard - which I think is strictly not allowed - and they validated my old copy for me with no trouble.

Mick