First check if you need to install the USB cable driver on your laptop. Did your USB to serial converter contain any CDs containing the driver? if not check the manual (if it was with it) and see if there’s a website from which you can download the driver from. (Nowadays some manufacturers prefer to give links to drivers instead of providing a CD/DVD with drivers). To see if you require any drivers open the Device manager (Right click on my Computer -> Manage and the Snap-in should display and click on Device Manager) then connect your converter and 8051 and see if anything from the list changes. If there’s an unknown device then you need to install a driver for it.
Try using ATMEL FLIP, it is a good program. Remember to hold RESET and press PSEN and continue on holding RESET and release RESET after a few seconds before clicking to flash the 8051 EPROM.
Hope this helps.