Guys, just telling him he needs to upgrade his computer without actually getting his specs?
First of all, posting your computer hardware and OS and stuff is always helpful when resolving stuff like this. Plus if you have any custom skins/models installed or not. Also, are you getting 20FPS? 10FPS? 40FPS? let us know how badly your FPS lag is, that can help us identify what the problem is.
Second, two things I suggest. One, go into your Steam/Steamapps/Username/CounterStrike folder and backup all of your maps and cfgs. Pretty much back up anything thats custom and you want to keep, put them somewhere outside that folder. Then go ahead and delete everything in Steam/Steamapps/Username/Counterstrike. Then relaunch the game. It'll have to download for a minute or two (might just say preparing to start or starting CS:S, just be patient). After that close CS:S and put all the stuff you backed up back in. I do this for TF2 after every single major update, it increased my FPS there by 20 and I'm simply assuming it has the possibility of doing the same on CS:S (individual results may vary though).
Second, download the program 'GameBooster' Its free, run it before running CS:S. It closes down all unnecessary processes your computer runs all the time (like Printer Spooler and stuff) and cleans and defrags your ram. I get myself an extra 5-10 FPS in TF2 after using that.
Also, make sure you don't have any internet browsers open, or if you do limit it to two or three tabs. When I have my normal 10-11 tabs open in chrome it lags my games.
After trying all of that, come back and let us know if your FPS has improved any?