using tv as a monior

[Yee]

I connected my PC to my TV so that I will be able to use
my PC as a DVD player with the TV monitor providing a
larger output. I connected it with a 2 way s-video lead
and a scart adaptor. However, once I scroll through my
AV channels there isn't any image on the TV screen.
Worse still is that when I played around with the display
settings in trying to get an image to appear on the TV
screen, my PC monitor wouldn't show anything when I start
up except for the desktop wallpaper. I guess I
accidentally set the TV monitor as my primary monitor (or
something like that), but what is worse still is that the
TV monitor still doesn't show anything to allow me to
correct the problem. I can only get my PC up and running
properly (showing icons and mouse pointer etc) when I
disconnect the connection from my TV.
How am I able to get my PC monitor to its normal state
when it is connected to the TV monitor (and not just
showing the desktop wallpaper) so that I can attempt to
alter the display settings to try to get an output from
the TV screen?