First you have to understand what you are trying to do (and I'm a little bit on the trailing-edge so I can hint at what's possible but the others will have to chip in with implementation details).
There are various ways to decrypt, and it depends whether you are mainly interested in StDef or HiDef. To play back HiDef from other devices your options are very limited - either another Humax box (eg HD-FOX) or a PC using Splash Player Lite are the only options I know of for sure. Splash requires the file to be downloaded first (won't stream). If you are not too bothered about HiDef then making StDef your default recording mode opens up your remote play options.
Now the question of whether you need to decrypt at all. As long as your recordings are StDef, any DLNA-capable player can access them from the HDR-FOX's standard server. The decryption happens on the fly. Running
auto-unprotect (which removes the protection flags from HiDef recordings in the background) will allow HiDef to be decrypted on the fly also, but as I mentioned before not much can play the resulting stream (an HD-FOX can play a HiDef stream even without auto-unprotect).
The custom software utilises the media server decryption (StDef or unprotected HiDef) to provide a decrypted download via the web interface - the result being a playable file local to the PC (Splash for HiDef, almost anything for StDef - eg VLC, WMC).
Now we move into territory I have not explored. Streaming has limitations which can be annoying - no transport control for example. Downloading before playback makes the experience much closer to playing it natively on the Humax (and VLC can play a file before it has finished downloading!). Setting up a network share (should) make the files playable as if they are local, as long as they can be decrypted first (because accessing by a network share bypasses the DLNA decryption capability). Hot off the press is
decryption in place, a process which tricks the media server into doing a decrypted download to itself, and thereby replaces the encrypted recording with the clear version.
For background reading I recommend
Black Hole's Trail Guide (it's a bit behind the latest developments, but shows how things have developed), and you should look at the
Decrypt In Place topic. More useful information will be found in the pinned topics in the HDR-FOX T2 section (I think we need to cross-index them in this section).
Update: I just read the decrypt in place topic, and it looks like it's a bit manual at the moment. I think it's still a better option than creating a virtual drive and using the RC handset to initiate decrypt-copy operations though. Personally I have no immediate need to decrypt except for archive/backup purposes, for which a bulk copy to external NTFS drive serves admirably, as my remote viewing is confined to a networked HD-FOX.