That is a question I get regularly from customers. Microsoft has a great site where you can find that information. Unfortunately this site is not easy to find, and a lot of people are not aware of this site. A good reason to promote it a little.

So if you ever get a question on this topic, go to http://support.microsoft.com/lifecycle/search/Default.aspx.

At that site, you can find also the details of the policy

Microsoft Support Lifecycle Policy

The Microsoft Support Lifecycle policy took effect in October 2002, and applies to most products currently available through retail purchase or volume licensing and most future release products. Through the policy, Microsoft will offer a minimum of:

• 10 years of support (5 years Mainstream Support and 5 years Extended Support) at the supported service pack level for Business and Developer products
• 5 years Mainstream Support at the supported service pack level for Consumer/Hardware/Multimedia products
• 3 years of Mainstream Support for products that are annually released (for example, Money, Encarta, Picture It!, and Streets & Trips)

Phases of the Support Lifecycle

Mainstream Support

Mainstream Support is the first phase of the product support lifecycle.
At the supported service pack level, Mainstream Support includes:

• Incident support (no-charge incident support, paid incident support, support charged on an hourly basis, support for warranty claims)
• Security update support
• The ability to request non-security hotfixes

Please note:

• Enrollment in a maintenance program may be required to receive these benefits for certain products

Extended Support

The Extended Support phase follows Mainstream Support for Business and Developer products.
At the supported service pack level, Extended Support includes:

• Paid support
• Security update support at no additional cost
• Non-security related hotfix support requires a separate Extended Hotfix Support Agreement to be purchased (per-fix fees also apply)

Please note:

• Microsoft will not accept requests for warranty support, design changes, or new features during the Extended Support phase
• Extended Support is not available for Consumer, Hardware, or Multimedia products
• Enrollment in a maintenance program may be required to receive these benefits for certain products

Self-Help Online Support

Self-Help Online Support is available throughout a product's lifecycle and for a minimum of 12 months after the product reaches the end of its support. Microsoft online Knowledge Base articles, FAQs, troubleshooting tools, and other resources, are provided to help customers resolve common issues.

Please note:

• Enrollment in a maintenance program may be required to receive these benefits for certain products

(source: http://support.microsoft.com/lifecycle/#tab1)