Once again, I encounter a problem with my PC which have been irritating for months!

After changing to a new CPU, I experience frequent disconnection from my connection. I've no idea what went wrong. mIRC and MSN was often disconnected. Can't browse website. Have to manually disconnect my connection and connect it again before everything resume back to normal but always after the reconnection my PC will become very slow. Internet Explorer takes quite long to load, take quite long to connect to mIRC and sometimes can't even log on to MSN. Why is that so?