Steam now support older mods :)

[vakuum]

If you want to play cs 1.5 and 1.6..

1.6 comes with steam.. ok..

copy your cstrike (1.5) folder from the old hl dir to your steamappz\username\half-life\ folder..
Rename it cstrike15 just to have some order..
if anyone has the cs 1.4 full installer they can just install it into the same dir and rename it cstrike14

then you could have
steamappz\username\half-life\cstrike15 and steamappz\username\half-life\cstrike14

This way it could be easy to play older mods or different versions..

The only thing is.. an older mod does maby NOT handle the new hl.exe that has been patched..