1. Exit Steam
2. Install a new installation of Steam ( just steam nothing else) to a different folder or partition. Eg. C:\My Steam\Steam or d:\Program Files\Steam if you have a D drive with plenty of space. Whatever as long as it´s not the same place as your existing Steam install.
3. Log into the new Steam install with your existing steam account.
4. Goto play games and in the " My games" section you´ll see that HL2 and CS:Source are not highlighted or ´lit up´.
5. Steam recognises what games you are entitled to by the account you have with Valve, therefore it knows what your allowed to download. It´s no different to those that had ATI voucher CD keys, were entitled to the software in the same way they are.
6. Double click it/ right click choose properties - whatever it will then let you install them through steam which will download all the files you need. This way you won´t have to portion of the retail install that tells the game to check you DVD is inserted.
7. Play the game
