Usually, this issue is resolved by rebooting the router, the PC and the DVR. The order shouldn't matter, but if it isn't resolved at that point, something is either not functioning correctly, or being blocked.
I was having this same issue and want to report back on my solution. It strikes me odd that nobody has mentioned this in any of these treads... my problem is that I was assuming that since bonjour and the TiVo service was allowed in my Windows firewall that it was opening the required ports. It was not!!
I had to open port 5353 for the TiVo to see my desktop.