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TV signals and VGA monitors are incompatible with each other. The basic fundamentals are theoretically similar, but in application they are very different. There is a big difference in the scanning rates, CRT phosphor pitch, and overall design specs.
Normal video signals you watch on TV screen are generally in composite video or RF (antenna) formats which VGA monitor does not have any suitable inputs. The HSYNC rate of TV is 15625 Hz (for PAL and SECAM standards) or 15750 Hz (for NTSC standard). The VSYNC rate is 50 Hz (for PAL and SECAM) or 60 Hz (for NTSC). TV uses interlacing so that it shows 25 or 30 frames per seconds and shows those in to parts: one field includes all even number horizonal lines and other all odd numbered horizonal lines. Interlacing is the process of scanning 1 frame in two passes - the odd (or even) lines first and the even (or odd) lines second. This enables the low framerate of 25/29.97 to display without flickering as much on a system where the phosphors don't glow for long enough to get from top to bottom of a frame in one pass.
VGA monitor can only take RGB signals with separate sync signals (HSYNC, VSYNC). Original VGA used around 31 KHz HSYNC rate and 60-70 Hz frame rate. The whole picture is normally drawn at each screen update (no interlacing normally, can be used if wanted). The HSYNC rates and VSYNC rates have increased because of increased resolutions and need for faster screen refresh rates for less flickering. You can in real life see signal and monitor specifications in the ranges of an order HSYNC=30-65 kHz and VSYNC=50-120 Hz (those are not absolute maximums, but quite typical values graphics cards output and monitors can take).
Because of different signal formats and different screen refresh rates the picture you send to your TV and the VGA monitor wants are totally incompatible. The TV signal can be conneverted to RGB format with some electronics (some equipment designed for TVs can even output RGB), even then you can't make a steady picture to VGA screen because the refresh rates of TV signal is not compatible with what VGA monitor is capable of handling.
To be able to show TV picture on VGA screen a considerable amount of conversion is needed to convert the picture coming in in one format to other format which VGA monitor wants.
There has been lots of question how to do the conversion in another way: using you VGA monitor as TV. Unfortuntely there is no easy way to do it and there are no circuits in the web I know how to do that. The reason that theare are no circuit projects on the web is that an NTSC->VGA line doubler is a non-trivial project unless you are an experienced digital/ analog designer - even with the new generation of chips on the market.
I have collected here some information on this topic because this conversion stuff is so often asked in newsgroups and I have been interrested in the topic. First read my How video is shown on computer monitor technical document and check the following links for more information:
The circuit diagrams that convert composite video or S-video signal to RGB signal suitable for SVGA monitor are quite rare. The link collection for links like this can be found at http://www.epanorama.net/links/videocircuits.html#tvvga.
If you know more such circuit on the web I would be glad to hear about it.