Is it possible to use an Android phone as network disk, so I can browse it on my TV? Is it possible to connect an Android phone to my TV with a USB cable, and then browse a network disk instead of the SD card? 

To answer comment questions: HTC Desire HD, TV is able to browse USB storage devices. At the moment I go to my PC, connect to my phone with the USB cable, and put a video file on the SD card. Then I walk to my TV and plug the phone into the usb port. Then I browse the SDcard to find the video file, and open it. Every once in a while I connect my phone to the TV, and then find out that I need to go upstairs to my PC again, because there are no video files left to watch. And then I think "If I only could use a network file explorer (like ES) and use it to browse it on my tv"... 

The TV supports none of the protocols since it has no network connection (no cable/WiFi). It has a USB port to browse usb storage devices.
It should be possible. You would certainly need root.

If you could mount your network share onto your phone, you could mount it under /mnt/sdcard/network_share. Then your tv would simply browse that folder. To your TV, it would simply look like another folder on your SD card.

This post shows how to mount nfs on your phone. You may need a special version of busybox.

busybox mount -o nolock,ro,hard,intr,vers=3 -t nfs 192.xxx.xxx.x.x:/your/nfs/share /mnt/sdcard/network_share