you might have already solved it by now but i'll post anyway..
try typing these into the console (to enable console go into keyboard options, press advanced):
- cl_showfps 2 <- this will tell you your frames per second. Usually anything less than 30fps i find unplayable. cl_showfps 0 to hide it.
- rate <- this sets the rate at which cs updates. Broadband gamers should use at least 15000 here (rate 15000). I'm on 1.5mb and i use 20000. 20000 = 20kb/s
- cl_smooth 0 <- this might help reduce the strain on your system
there is also something called Vertical Sync (vsync) which can be found in your graphics card options. When set to 0 or off, cs will get higher fps and might run better on your system. With vsync on, i can only get 60fps max on cs1.6, never checked on cs source.
hope this helps