In a Windows Vista system if you have more than one browser installed, then you can set your your preferred browser as a default by the following ways.
Listed below are the steps for making ie8 and default browser using default program access.
Click on Start and select Default Programs, and click on the Set Programs access and computer defaults.
Choose the configuration as Custom.
And click on Internet Explorer radio option
By this way you can make ie8 as a default browser in Vista.