Forgetting IP Addresses?

I have a machine on a network that is set to receive a dynamic IP from the router. The Router was set to assign the same IP address every time a couple of weekends ago. The Monday morning the user couldnt access the web or get email so we changed the router to assign any address from the pool and the user could get online and get mail without issue.

A couple of days later, same problem. Got the user to open a command prompt and go through the whole ipconfig /release then ipconfig /renew thing. Problem fixed. But since then every couple of days or even sometimes twice in the one day he has to release then renew his IP address.

I have not come accross this before so I'm not sure if the problem lies in Windows or in the NIC of that machine. The fact that when its renewed everything works fine leads me to think that the NIC is fine and the problem lies elsewhere, but where?

Can anyone help me track this down?