VGA video (which is what comes out of a PC) and composite video (which is what TVs and VCRs use) are different formats. For the complicated technical stuff, see here:
http://www.tkk.fi/Misc/Electronics/circuits/vg a2tv/video2vga.html
(That page is actually about doing things the other way round, i.e. showing a picture from a VCR on a computer monitor, but it still explains the differences).
You need a device to convert between the formats, like this one:
http://www.maplin.co.uk/Module.aspx?ModuleNo=3 5258&source=2001&doy=search
(There are cheaper versions available. Try searching eBay for 'pc to tv').
Chris