hmmm, sounds like you have bought a duffer. Is it an "easycap" or "ezcap"?
You may be lucky and it will be the capture software but I doubt it seeing as you have tried on 2 different spec pcs.
Try this:
Download this capture software (free, stable and good)
Debut - xp/vista/windows 7
http://www.nchsoftware.com/capture/index.htmlTry capturing. If it's doing the same it's the unit. Send it back or bin it.