hi and welcome to the forums
ok so you have discounted it being the operating system then if you have reinstalled..
can you provide any pings are traceroutes to the sites that you are trying to connect to?
start > run > cmd > tracert
www.whateveraddress.com
ping
www.whateveraddress.com
i would also suggest that you try and connect the pc directly to the cable modem to see if your issues continue so then we can directly rule out it being the router as well