So this seems like a router problem, but it started happening since I installed Windows 7.
I often lose the network (networking icon gets exclamation mark) and need to reboot my router which fixes the problem temporarily.
I am using the Cisco Linksys router WRT160N V3.
Running Windows 7 Ultimate N 64 bit.
Also, rebooting solves the problem, so it's probably not a router issue.
The computer is connected to the router via cable.