In case you find yourself in a situation where you need to get the SID of a user account here are a few ways to do it.
Local account with PowerShell – http://technet.microsoft.com/en-us/library/ff730940.aspx
PS > $ob = New-Object System.Security.Principal.NTAccount(“test”)
//Where “test” is the local account name
PS > $sid = $ob.Translate([System.Security.Principal.SecurityIdentifier])
//Now $sid contains the value of $ob translated to it’s SID
PS > $sid.Value
//Just echo the value
You can grab a domain user the same way by replacing “test” above with “domain\test”
There are some other great PowerShell and SID resources at http://blogs.technet.com/b/heyscriptingguy/archive/2010/10/11/use-wmi-and-powershell-to-get-a-user-s-sid.aspx
If you are just concerned with a SharePoint 2010 managed account you can also use;
$(Get-SPManagedAccount -Identity “THE SERVICE ACCOUNT”).Sid.Value
And if you’re still not super comfortable with PowerShell this is where you can find it in the registry.
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion \ProfileList
//Under the ProfileList key you will see the SIDs and you can check the value for ProfileImagePath to see what username is ascociated with which SID. The main drawback to this method is that the account must have logged into the computer at some point to have a profile value.