It sounds to my non-programmer ear like you have a memory leak. (That means that something is asking the OS for more RAM to do a task and then not handing it back when the task is done.) I mean, I could be totally wrong, but that's the first thing that came to mind. Can you use the default user profile to hit Google and do a search on problems like yours?

As for the origin of the problem, if it happened the exact same day as the wifi installation, I'd say the Charter people are full of it. What happens if you shut off/uninstall whatever software you set up to go wireless? Reformatting your hard drive is what you do after you've tried everything else. They're probably just reading from their My First Tech Support Call manuals, which tell them to jump straight from the quick and easy stuff to the drastic with no time in between, because time is money and they don't want to give you any of theirs if they can help it.

Finally, do you have the most up-to-date virus software? People Creepy losers make new ones all the time. Have you looked for spyware? How about a zombie program? Spammers love to find undefended computers connected straight to the internet without a firewall (do you have a firewall set up?) and grab control of them just enough to start using them to send scaaaaads of spam.

Good luck. Computer problems suck big time.