One of the (Windows XP SP3) workstations of our Windows Domain could not access internet anymore, this problem suddenly happened overnight. The domain controllers (there are three of them) are all running Windows Server 2008.

First I compared the output of ipconfig /all on the faulty workstation with the output of a working workstation and it was just fine as it had always been. In particular the default gateway was correct and always remained pingable from the faulty workstation.

I guessed that something was wrong with the DHCP service and I restarted the DHCP server service on all of our three DCs as well as the DHCP client service on the faulty workstation. This didn't solve the issue.

I then thought of renewing the DHCP lease with ipconfig /release and ipconfig /renew and here is my first question: why did this never work? The same IP address (192.168.0.45) kept being assigned despite all my attempts to renew it (note that all our workstation are getting their TCP/IP automatically.) Even by leaving the domain and changing the computer name the same address was yet again obtained...
Anyway I then proceeded to switch the TCP/IP configuration for that machine manually to another free valid IP address (192.168.0.41)... and then the internet access came back!
I then cleared any traces of the previous IP in the DHCP leases list and in the DNS tables of our DCs and, after setting back the TCP/IP configuration to 'automatic', finally, the new lease would be granted (192.168.0.41) alongside with the internet access.

My second question: what went suddenly wrong with the original IP address?