To your 1st question... there is no need to re-install the router. Go to its web page 192.168.1.1 (maybe) & check or modify the settings there. Best suggestion is call the 1-800 # for your router's Help Desk Support & have them assist you w/ this.
I'm not a big fan of Best Buy but, it stikes me as unlikely they would have left your router unsecured.
To your 2nd query... NO, that will not be why your computer is running slower than usual. Factors contributing to that would primarily be things like accumulation of 'junk', unnecessary files, malware or unwanted or unneeded programs running in the background.
Cheers,
Drew