Try without Norton first. God I hate Norton.... If that doesn't help, try this:
If you know what an Autoexec.cfg in CS:S is, then you have probably made one, and you need to back it up, and if you've made extensive changes to controls or anything of that nature, you need to backup config.cfg, also. They are located in (SteamDirectory)\Steamapps\(yourusername)\counter-strike source\cstirke\cfg
Make sure to back them up to somewhere such as the desktop, and don't delete them. Then, delete the ENTIRE cstrike folder. When you start CS:S up again, it will take a while to re-build this folder. This boosted my FPS by 20, and I believe its due to all the crap some servers make you download.
Let me know if this doesn't work.
Now if you don't know what an Autoexec is, then let me know and I'll get you an FPS boosting autoexec. It's a small text file (but instead of .rtf or .txt, it's .cfg) that has commands that CS:S uses. CAL (Cyberathlete Amateur League) and CEVO both have autoexecs that are legal for their competitions, but if you don't plan on playing those two, let me know.
Good Luck!