Network - Main computer; no internet connect?

Hello,

My connection set-up:
- I have one main computer (Windows 7, Acer) connected to my modem and wireless router.
- One laptop and PS3 connecting to the wireless router for internet access.

The problem is, that my main computer cannot access the internet, but every system using the wireless router for internet access can.
On the main computer neither IE or Firefox can connect so I don't think it's a browser problem.

- First of all I thought that I may have changed one of the crucial services by mistake, when I was playing around with them as I usually do. However, this was not the case. Even when I changed them back, the problem still remained.

- Next I thought it may be the Ethernet cable connecting the modem and the main computer (with the problem). Perhaps it was old or faulty? That wasn't the problem either.

- I've tried using Windows' Troubleshooter too and it said that it can't find a problem.


I made a few other small changes that may have cause the problem originally, but I can't think what. :/

Can anybody else think of some factors that may be causing this problem?

Any and all help is much appreciated.