2408WFP monitor to PC using HDMI cable

I just bought a new PC (Windows 7) and am hooking it up to my existing 2408WFP monitor. The monitor was connected to my old PC (Windows Vista) using a VGA cable. The new PC has only HDMI and DVI ports, so I bought an HDMI cable, connected them, and assumed the monitor would auto-detect. It did not, but is displaying a screen message indicating the VGA cable is unhooked. How do I get it to use the HDMI ports/cable?  Kindly advise.

TIA