What exactly are you trying to achieve? Turning off WEP is not really a good idea, unless you are turning ON WPA or doing some specific troubleshooting. The only way to get your WEP key is to have access to the router, and if Comcast didn't leave you a password, then they are the only place you will be able to get any help, short of resetting the router (which, in the case of a Comcast Home Networking setup, is probably inadvisable).
I'm with the others here, call Comcast. that's what you're paying them for. If that's not working as well as it should, then do yourself a favor and ditch the Home Networking service and get your own router and be in complete control.
If you are bound and determined to get this yourself, get the default gateway address from your "ipconfig /all" output, and point your browser at that IP address. The default login userid is "Comcast" and the default password is "1234". If they were changed during installation (which is good security practice), then you'll have to dig it up if the tech gave them to you, or call Comcast to get it or have it reset.
Once you have the gateway's config pages in front of you, simply go to the wireless security page and read the WEP keys right off the page. I don't have exact instructions because this device as it comes from Comcast has a custom firmware and the web pages it provides are different than the normal Linksys ones.
If you're turing off WEP to turn on WAP that's a good idea. WEP can be broken in under 2 minutes by someone that knows what they're doing. I would suggest that you stop paying for Comcast HN and just set it up yourself with a linksys router and be in control - changing WAP keys every so often. But if you must pay for that service be very sure they set you up on WAP keys.