As said, your computer screen is far higher in resolution then a TV and its a progressive screen where as a tv is interlaced...
for NTSC(Canada, US, Japan has there own NTSC-M or something) this is normally 640x480(720x480 for DVD's)
for PAL(Think Europe) 640 x 576(720 x 576 for DVD's)
Now on to interlaced video...SD TV refreshes every second line(easy way to say it) so it can make things look wrong on a monitor. There is software that can de-interlace, but it may cause a slight delay(or even one second on a hardware TV card)
If you want to try look for a program called descaler. See what it does for you, may just make it look good enough to play, but the slight delay(varies with cards) may not make some games fun.....never hurts to try...http://deinterlace.sourceforge.net/
A bit more info on interlaced video(well a picture
Make sure you click it and make it full size
First if Interlaced - this will be seen during motion as each line is refreshing....
Next is De-interlaced - Looks better....If I had good tv signal it would be allot better, but you get the idea
Last is Field Duplication - Looks ok, not as good as de-interlaced. This looks horrible on cartoons and some games. Many low end TV cards do this as it keeps the cpu utilization low....