Check which folder you're actually using and you can pretty much delete the other one. They changed the directory some time back from Program Files\Steam to Program Files\Valve\Steam. So, I'm pretty sure the 'Steam' directory in Program Files can be deleted. You can first transfer some downloaded maps and config files if you want to.
Also check your Valve\Steam\SteamApps folder. What GCF files do you have? Those take up the majority of the space. If you need everything there then you can look in your cstrike folder for extraneous files to delete including *.dem.
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BTW, I have CS:S, DOD:S, HL2, HL2DM, CS 1.6, Natural-Selection, Dayhard, Garry's Mod, Smod, Hidden, the SDK, and about 50 custom maps across them all. It totals 16GB in my valve\steam directory. CSS by itself I think is under 5GB. It uses some Source engine caches, but I'm not sure which ones. The folder itself is 1.7GB.