how can you "think" its just your computer? did you actually do the test the pular recommended? If you did, they you would KNOW if its your pc or your isp connection.
when you say lag spikes, is your in-game latency jumping into the 100's, or are you just getting low fps? If you are getting latency spikes on your pc ONLY, go through your installed/running programs and processes and begin turning them off. something installed on your pc is most likely causing these spikes. if all else fails and you cant find whats causing your spikes (if it is indeed on your pc...no offense but i dont trust your troubleshooting skills for a second), i would just format. its the universal fix
as spidey stated, creating threads with subjects of "HELP ME" and providing relatively no useful information goes a long way toward getting your questions ignored. i guess im only responding because i too play cs:s...recently quit bc of issues with cal and cevo, as well as life.
edit: also, when your in cs:s, go to the console and turn on net_graph 3. it will show your fps, in/out kbs, choke, ect. i would also recommend settings you cmd rates in game. apply the follow commands to your autoexec file located in your cfg folder:
cl_interp 0
cl_interpolate 0
cl_lagcomp_errorcheck 1
cl_cmdrate 100
cl_updaterate 100
cl_rate 20000
cl_smooth 0
rate 25000