Just sharing with fellow 56kers-
If you have downloaded the 300mb version of Steam and realised that the installation fails because one of the files gives a CRC error, then don't give up because there is a way to get it to work. Read!
Once the CRC error comes up, just let the installation fail, however don't delete your Steam directory because the CS cache is in there and the installation just couldn't install the actual Steam client. So you need to simply download the 500kb Steam client w/out any cache and once you install that you'll be able to install CS from the cache file (it wasn't corrupt, remember?) and it'll work fine!
Before I knew this I was gonna hang my head in shame and start the 700mb Steam d'load (I'm on 56k), but then discovered this and am now happily fraggin' some ass!