It is not a virus, nor did you actually change it yourself.
The prompt by default begins with the hostname of your computer. The host name is found by doing a so called "reverse lookup" in DNS of the IP address of your computer.
Every time you connect to a network, you'll get an IP address. The provider of that IP usually determines what the reverse lookup will say. Usually it is your internet provider.
Your ISP has thus chosen "dhcp-xx-x" as the hostname. It probably has some internal meaning to them.
DHCP is the name of a protocol used on networks to automatically deliver IP-addresses (and other configurations) to computers. Seeing this in your hostname typically means that your ISP used DHCP to deliver the IP and accompanying configuration to your computer.