What does ifconfig show?
dmesg | grep eth

Look for things like 10 mbit or half duplex etc...

Although those things are for hardware checking... and if apt-get works at acceptable levels... your hardware/device driver is probably fine.
Doing a apt-get update is fine. If you do an apt-get upgrade, you will likely trash the installation. So don't do that. If you have already... well I would save the /home files you need, and re install. I did after apt-get upgrade messed mine up.