are you getting anything on screen at any time?
sounds like mac is right ... no signal ..= no signal
however ... before you buy a new card ....
1 check that the lead is plugged in to both ends - pc and monitor.
2 Check that the card is seated in the mobo (or is it onboard graphics?)
3 is an odd one ... but I've seen it enough times to mention it. Check that the power is good
I carry a psu for testing in my toolkit ... but
http://www.maplin.co.uk/module.aspx?ModuleNo=4 5500&doy=19m4