for non clustered instances, these two techniques are working great for me, as long as i have RDP to the machine:
without stopping and starting the services, byusing sysinternals psexec to impersonate NT AUTHORITY\SYSTEM and gain access:
http://sqlblog.com/blogs/argenis_fernandez/archive/2011/07/10/think-your-windows-administrators-don-t-have-access-to-sql-server-2008-by-default-think-again.aspx
using a script prrovided by microsoft, as long as bouncing the services is no problem:
http://archive.msdn.microsoft.com/Project/Download/FileDownload.aspx?ProjectName=addselftosqlsysadmin&DownloadId=9198
i've used both, and not had any issues unless the instance was clustered.
Lowell
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