Changing routers, one computer won't see the router

I recently had fixed wireless internet installed with two PCs, and they game
me a different router than what I had, but I have decided I don't want to
keep the wireless internet, but now I try to plug one of my PCs into the old
router, and it doesn't see a connection, at all, like it did before the
wireless internet was installed.. but my other PC sees if just fine no
matter which router I plug it into... does anyone know what could be causing
this? When I plug this PC into the wireless router, it shows a connection
just fine, but I will be getting rid of the wireless in a couple weeks and
want to be able to hook it back up to my regular router. Before wireless was
installed they both worked just fine on my old router, and no software has
been installed on either PC.

--
"I contend that we are both atheists. I just believe in one fewer god than
you do. When you understand why you dismiss all the other possible gods, you
will understand why I dismiss yours."