problems with new wi fi

This one bewilders me. We've been using xplornet wi-fi in our home for 2 years. My wife has a pc and I have a macbook >> no problem. Recently, Xplornet upgraded our system to be much faster (4G) I think. Since that day, my macbook runs v-e-r-y slowly if I can get on at all. It's not the internet, because the PC runs beautifully. I had my macbook serviced and it ran quickly at the shop, but as soon as I got it home: boom, slow as molasses.
The PC is wired into the router and I use the airport on the mac. Has anyone heard of this difficulty before? Will the fix be as simple as getting a new router? Currently, we're using the same router (D-Link) as we've always used.
Any ideas or experience would be appreciated.

Once you are home, if you turn off Airport and thus disconnect from the Internet, does your Macbook run as fast as it did before? Is the slowness only seen when browsing the Internet or doing things requiring an Internet connection?

internet and mac

Hi, thanks for your interest. Yes, the mac is fast except when trying to use the internet and only here at home. We were in a hotel for a few weeks recently and it connected and browsed the net beautifully. It seems to be a problem with the wireless part of our modem. Although another PC connected wirelessly just fine.