You need to buy a KVM or KM switch also called a Keyboard/Video/Mouse switch which will allow you to swap between two or more PC using only 1 monitor.
Good companies doing this are:
Belkin KVM
Cables to go KVM
Tripp Lite KVM
IOGEAR KVM
KVM Switches Online:
http://tinyurl.com/l5n64
For example of KVM Switch:
http://en.wikipedia.org/wiki/KVM_switch
For example of KM Switch:
http://tinyurl.com/f5rln