You need a 'KVM Switch' (
http://www.webopedia.com/TERM/K/KVM_switch.html ), these allow you to connect your Keyboard, Mouse and Monitor to 2 computers and you switch between the 2.
Go to your local PC Store and get one priced up.
They are available in numerous configurations ...
DVI - VGA
PS2 - USB
Muliplt DVI & VGA
With and without sound.
You will probably find that Belkin is the best known brand.