Computer with network but without Internet (LAN) (many things done)

Hello

I have got a big problem with a PC.

SCENARIO: Local area network with 20 PC with Windows 7 SP1 and 2 server with WS2003 (SBS). The Internet is provided by ethernet cable but the router gives Internet by WiFi. Also there is a big switch to connect all the computers.

PROBLEM:

One PC without any reason lost the Internet connection but conservates the network access (this PC see the shared folder, other PCs, etc).

Things done to solve it:

Plug and unplug the cable.
Renew the dns (ipconfig /release, ipconfig /renew).
Execute a professional antivirus (AVAST) (it saw 30 infections and they were removed)
Execute CCLeaner (clean space, clean registry) (many times).
Execute Malware Bytes (delete one or two malware files).
Install new ethernet card (same problem, the machine has got network but no internet).
Update the ethernet driver.
Update the BIOS driver.
Remove many unneccesary applications.

Things to considered:

I plugged my laptop (W7) with the cable and then laptop had got network and Internet.
I take the PC to bring it to my house and with a WiFI dongle had got good Internet connection but in the office, the connection taked by the WiFI is really very slow.

Questions:

Could the router be the problem? But the laptop connected without any problem with the same cable.

Thank you.

I will do that things and I'll update with the problem. But I don't understand why browser's proxy setting can impact to all the Internet connection. I see the yellow signal (exclamation) on the icon of the ethernet in tha tray task.