ok... you have a couple of options, which would all be sort of expensive
there is a vga cable on this website... http://www.nintendo.co.jp/wii/accessories/index.html
i don't know if they will ship to america or if they have it on american site
you can get a capture card which would be the easiest option, unless it allready has one, did you build your pc? if you did did you put a capture card in? if you bought it, what pc did you buy?
and you can get an expensive thing that will cost more than an hdtv to convert A/V to DVI