I think below will give the expected result:

-- Create Temporary table to store the data

Create Table #SQLInstances

( Value nvarchar(100),

InstanceName nvarchar(100),

Data nvarchar(100))

-- Read Data from Registery

Insert into #SQLInstances

EXECUTE xp_regread

@rootkey = 'HKEY_LOCAL_MACHINE',

@key = 'SOFTWARE\Microsoft\Microsoft SQL Server',

@value_name = 'InstalledInstances'

Select InstanceName from #SQLInstances