Under certain conditions a PC running Windows Vista is unable to obtain an IP-address via DHCP from the Internet Service Provider (ISP). The problem occurs if the ISP is running a non-Microsoft DHCP server. Among ISPs it is common to work with, for example, ISC DHCP-server which is the most common DHCP-server in the world.
The problem and a possible solution is described in the following Microsoft article:
If you don’t want to edit your registry another simple solution is to buy a simple broadband router (internet sharing device) that does Network Address Translation (NAT). By doing this, the broadband router is obtaining the IP-address from the ISP, not the Windows Vista PC. In addition, you will be able to connect more than one PC to your Internet connection and those PCs will be protected by the firewall that most broadband routers have builtin.