this is my go-to script for this issue:

Login to the server itself, make sure you have local admin rights on the operating system side of things.

Now simply download and run this script from an administrative command prompt window:

https://gist.github.com/wadewegner/1677788

you are prompted for which instance(blank fro the default instance, otherwise put in the name)

it adds you, the current logged in windows user, as a sysadmin to the SQ Linstance. it stops and starts the service, so mission critical servers should wait until a maintenance window when you can bounce the service, but it's a fast process.

repeat for each SQL instance on the machine you need to take over.

I've used this on nearly every server I have, in a recent takeover; network guys gave me local admin to a box, and i took over the SQL side of things.

the original copy of this script, direct from microsoft was here, but has been moved because they retired the archive. I found a copy by searching for "addselftosqlsysadmin"

http://archive.msdn.microsoft.com/Project/Download/FileDownload.aspx?ProjectName=addselftosqlsysadmin&DownloadId=9198