the issue may be hdcp, if both the pc and tv arent enabled you wont get a picture. try to connect dvi to dvi or if you havent got dvi try an adaptr more...
JVC AV-48WP74 48 in. Rear Projection... - Troubleshooting
compatible with the picture signal of a personal computer. Use a DVI-D single link 19-pin cable (commercially available) in order to digitally connect DTV decoder Quick Setup Connections Connecting to a Digital TV Receiver w/HDCP 17 more...
JVC AV-48WP74 48 in. Rear Projection... - User Guide (page #17)