If you happen to install Microsoft SQL Server under a Windows account you later want to change to another Windows account you will have to login to Windows using the account it was installed under (Go to Control Panel>Administrative Tools>Services to find this out). You should then stop all SQL server services (MSSQLServer, SQLServerAgent, Microsoft Search, Full-Text Search, Distributed Transaction Coordinator etc). In Control Panel, click on Administrative Tools and then Services. For every SQL service you have installed you will need to view and edit the logon properties, changing the account for each one. A reboot should then complete the process.