To safely change the account that SQL Server runs under, select the server's Properties sheet within Enterprise Manager. Switch to the Security tab. In the Startup Service Account box, make the appropriate account selection, either System Account or This Account. If you want to change the account to a domain user account, you must first create a SQL Server service account with administrator priveleges and be logged in as the account that you're changing SQL Server to run under (i.e., domain\user_account). If you're not logged in as the domain user you're setting the account to, you'll receive the following error message:
A required privilege is not held by the client.
If you are running Full-Text Search, do not change the services account using the Control Panel's Services applet. Using this applet to change the account may cause Full-Text Search to stop functioning. You will not be able to rebuild or repopulate the index until you set the account as described above. When the account is successfully changed, rebuild the index and Full-Text Search should function properly.