Screen size

I have a samsung 19" widescreen monitor which has worked perfectly with all
games (Call of Duty 4, BF 2 etc) up until i recently installed a product
from Cucusoft to convert AVI into DVDs.

Now the system boots up and plays games in a square 19" format and not
widescreen. The desktop however still fills the widescreen - as do all normal
programmes ?!

Is there a setting or something i can do to get my full screen filled again
for games and bootup ?

Thanks