Problems with ethernet connection on Acer Aspire 5740G laptop

Hi!
I own an Aspire 5740G that has an internal ethernet card - Broadcom netlink ethernet adapter is its name under device manager - (other specifications of my laptop can be seen below or given at request). I used to connect via RJ45 cable to my dorms LAN that also provided high-speed internet.
The problem started when I decided to update my original Sun Virtual Box installation to one more up to date. The software informed me that my network setting would be reset and I agreed I have never had problems with this in the past. Then the network connection was broken. The device was recognisable, the cable was shown to be connected but no data was sent or received by the ethernet adapter. Moreover, in LAN properties dialog the second tab ("Authentication") disappeared. As a result not only I was not able to connect to the internet, but also had no LAN. Nothing worked (eg reinstalling ethernet drivers, installing drivers update, uninstalling VBox) except System Restore, that reverted everything to normal. A few days after that happened again, after a windows update, the system was reverted again; after the installation of the SP2 (and some other updates too) for the newly-installed office 2007 it happened again...
I really don't know what to do as windows diagnostics and driver re-installations don't seem to help. Is there a way to fix this as it happens or I just have to do a system restore every 2-3 days?
Thank you very much for your time!