As long as you have a valid Half-Life CD-Key, you can freely play any Half-Life game and its mods.

Now that's been cleared up, I have finally managed to play Steam. I left it on the Validating Resources screen, and it worked eventually. Just let Steam be and you will get in, ponly need to do it once.

When I got on though, it was incredibly laggy. Happening in both HLDM and CS. The answer to this is to just leave Steam alone and let it sit there and download all the content it requires (or get the installer with everyhting you need). Even if one game has been downloaded and verified fully, other mods may be downloading in the background, reducing bandwidth and spoiling your game. If you give it some breathing space however, it will get it all sorted, you can fit in some time to watch that DVD thats been lying on the shelf (a VHS for those behind the times I guess ) making sure anything like standby is disabled under Power Options in the Control Panel. When its all ready, you can get the best out of Steam and not have any lag issues or the thing going wrong all the time.

Hope that helps, try Steam after letting it do it's business and you will find its not so bad.

IRC Support can be found on west.gamesnet.net on #steam
/msg SteamBot with your question.