Try using Internet Explorer, not your whatever browser.
Save a website to your hard drive for offline testing.
Then try starting up your computer and removing your wireless card, disconnecting any landline communications, etc. Then open your saved website in one of the crash-prone browsers. Do you crash?
I'm not sure the above steps will help much, because I don't think it's likely you'll crash when reading offline.
Have you done a full cold reboot on your entire system at the same time? I mean actually unplugging your wireless router, cable modem, computer, printer, etc. and plugging them back in for startup.
Then try resetting your network connections. I forget how to do this.
Try uninstalling your browser and Steam (and make sure they're gone from Program Files) and reinstalling fresh.
Then try resetting your modem to factory specs. There should be a little button. Before you do this, make sure you have all the documentation for the modem and router printed out, and you have all your passwords and stuff. Ideally, get a friend who has another internet connection at hand to help look something up if you can't.
Only after these would I consider something so dramatic as reinstalling the OS.