Lets start with the basics:
Are you running an antivirus or p2p programs in the background as these are known to use system resources and bandwidth, if so then disable them.
Is Steam downloading an update in the background? if so, disable it.
What are the settings in your config.cfg file for the following:
The defaults for these settings are:
- rate 20000
- cl_updaterate 101
- cl_cmdrate 101
The average setting (for most players is:
- rate 18000
- cl_updaterate 60
- cl_cmdrate 60
But you can play around with them, generally RATE is set between 15000-20000 and CMDRATE + UPDATERATE are the same as your average FPS, so 80FPS would give CL_CMDRATE 80 + CL_UPDATERATE 80.
A good place to start with the UPDATERATE + CMDRATE is 30 and work up to 250 and see which value gives you the best results.
Hope that helps.