Currently I bought a computer with Vista.But when I want to write equations it doesn't have an equation editor.what should I do?

It is part of office 2007 now.
See the links below.

Check it out here
Download MathML .NET Control Free Trial - MathML .NET Control is a Equation editor component designed for the .NET Framework. It's an Equation editor for all users... (http://www.download32.com/mathml--net-control-i9881.html)

See this question
https://www.askmehelpdesk.com/computers-beginners/windows-vista-microsoft-office-2007-microsoft-equation-3-0-a-141710.html

Windows has NEVER included an equation editor. There is one built into MS Word however. Or you can purchase a third party product.