hi Ritesh,
1> to use usb to serial cable u should download proper driver for ur o.s.
i had same prob once,then i used s/w from BAFO.now its working fine.
2>while using flash magic,check option COM port in 1st step.that should be the same of usb-serial com port.ok.
now check proper device.( like P89V51RD2 ) .again if its not working c if sw of power supply( of 5 volt on interfacing card ) is on.make that OFF.load hex file in flash magic.now it will show msg ‘set device in isp mode’, now make that sw ON.n u’l program ic. ( or even that p.s. sw is ON. u can dung hex code while pressing reset key on card )