Windows claiming the IP address is in use – ARGH!

I've bought myself a new ADSL2+ modem this week, a Netgear DM111P, because my existing ZyXEL R-660R-61, which I've got from my internet provider, had some sort of bug - It denied the connection to certain IP addresses after a while, and sent back "Source Quench" ICMP packets when trying to contact the addresses. The [...]