A Three step solution to you fixing stream!
SOLUTION IF the first step didn't work and btw this worked for me.
Go to key: HKEY_CURRENT_USER\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers
Look for a entry with your path to steam.exe
Delete that entry
If you dont find it there try HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\AppCompatFlags\Layers
Okay now for 2nd step; Mainly your just deleting another folder in regedit; and for the proffesionals out there feel free to test as this worked for me.
HKEY_LOCAL_MACHINE -> Software
You should now see a folder named "Valve". Delete it, the hole folder (left click folder "Valve" right click and delete, confirm deletion)
Okay now close the Regedi; go to steam and click properties and click compatability mode click "Change settings for all users" un-check compatability mode and Run as administrator. Apply and close it. Stream shall now work for you. I do hope the administrators of this website put this up cause it worked for me, as as a vista user i can proudly say nothing is ever easy >,<