A word of timely warning here - I discovered this problem last weekend.

Some things in Hammer do not like the use of "blahblah@blah.com" in the path.
So if, for example you were to select a sprite from your Steam CS directory then the map may not work ("bad mod_num for game" or some such error).

FWIW, I would strongy recommend using your old C:\Sierra 1.5 directory for map-making and building with Hammer - just copy any new stuff back into the approprate area here and it all works with no problems.

...you do still have your 1.5 CS don't you ?
(if not I guess you could just copy the counter-strike directory to somewhere else like c:\hammer\cs and it should work - I'm pretty sure its the "@" symbols in you Steam CS path that f00ks up Hammer).

Also, go and get the latest ZHLT (Zoners Half Life Tools); and configure Hammer to use for compilation - 99.9% of map-makers will agree they're the best.

Do a search for Hammer configuration tutorials or something like that and you should get things sorted.
If you're still having problems then let me know and I'll try to help out when I'm around.


P.S.
The Handy Vandal's Almanac is a great starting point.
http://www.karljones.com/halflife/almanac.asp

HTH