Tried Re-Initializing The Modem/Router?
First, turn OFF the computer, then turn OFF the router, then turn OFF the modem (if separate from the router).. Wait ten minutes. Now turn ON the modem and wait two minutes for it to re-initialize with your ISP.. Then turn ON the router (if separate from the modem) and wait two minutes again.. Now turn on the computer and see if things are better.
And although you mentioned changing the password on your wireless internet, you didn't tell us whether YOUR connection is wireless your whether you're using a ethernet connection to the router.. If the above step doesn't fix things and you're not using a wired ethernet connection, you might try it as a test to see whether the wireless connection is indeed the issue.
Hope this helps let us know more.