I can't believe the people that installed the wireless security cameras didn't say anything about it potentially or probably interfering with your 802.11 home wireless computer network. Wireless networking is pretty pervasive these days in homes.
Edit: I read this thread again, and I find it humorous since my job right now is tech support. I deal with the same kind of thing on a daily basis, where a very key piece of information is omitted because the caller just doesn't know any better. You'll spend an hour trying to figure out something out and it just isn't making sense why it's not working and then they'll say, "We had someone out messing with the wiring in the store this morning, do you think that might have something to do with it?"
The worse I ever had was trying to figure out why a store was getting a modem error when trying to sent their sales information over the dial-up link. I did everything: cycled the power on the modem, rebooted the computer, reseat all the modem's connections, try the modem in a different phone line, tried a different data cable, tried a different phone cable, resent all the modem's init strings, reinstalled the modem driver in Linux, restarted the ppp0 interface, verified the correct DIP switches were set... All this took like an hour and a half. It wasn't the modem itself because they happened to have two for some reason and we tried both.
Finally, exasperated, I said, "What lights are lit on the modem right now?" Answer? "None of them." I bolt up: "Have you seen which ones were lit earlier? What were they?" "I don't think any of them were ever lit." There is always at least one light lit if they have power. The power cord wasn't plugged in.
You learn real quick from stuff like that. I always check lights first to make sure they have power. I always make sure that if I talk about the computer that I quickly specify that the screen is not what I'm referring to when I say "computer." I always make them replicate the error and read it verbatim rather than letting them generalize what they thought the error was. I always ask if anything has happened somewhere around the time the problem started happening.