For users with Windows Vista Ultimate or Business editions:
>>Right click "Computer"
>>Click "Manage"
>>Double click "Users and Groups"
>>Click "Groups"
>>Right click "Administrators"
>>Click "Add to group..."
>>Click "Add"
>>Click "Advanced"
>>Click "Find Now"
>>Double click "Local Service"
>>Click "Ok"
>>"NT Authority\Local Service" should show up in the list now
>>Click "Ok"
>>Close Computer Management and reboot.
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For users with the Home editions of Vista:
Go to the command prompt (hit run and type in cmd.exe)
and type this in:
net localgroup Administrators /add Local Service
then you can type this in:
net localgroup Administrators
and if LOCAL NT AUTHORITY/SERVICE is listed, then it worked. Now go reboot.
source: http://www.facepunch.com/threads/479952-HL2.exe-has-stopped-working-fix