Meehlimo:
Hello and welcome to the forums.
TorrentG's suggestion of uninstalling the AVG firewall product is a good and sound way to move the diagnostic process steps along and it costs nothing but a few minutes of your time. You can always promptly reinstall it later once you've managed to resolve your current issue. As TG suggested, Microsoft Security Essentials has been found to be a satisfactory substitute for most of the top tier AntiVirus / AntiMalware products so it might be worth your efforts Link Removed due to 404 Error
Although, I haven't seen the autonegotiate issue with some Gigabit Lan Cards happen intermittently (usually it either works or it doesn't) you may try this and see if it works for you.
type ncpa.cpl...