If you have got a Windows Vista and Windows 7 dual boot PC and you want to the default OS to Windows Vista, you can modify the settings by using the Startup and Recovery options.
The Startup and Recovery dialog can be accessed through Control Panel -> Systems and Security -> System -> Advanced System Settings.
In the System Properties, select the Advanced tab and click on the settings button under the Startup and Recovery
Under the System Startup section, there is a Default operating System drop down control, using this the default OS can be set to the preferred OS
Similarly, you can specify the time in seconds for displaying list of operating system using the Time to display the list of operating systems