Probably not possible I'm afraid, unless the PC monitor has an HDMI input (some do, some don't), in which case you may just be able to connect the box to this (though may run into possible problems with HDCP, but easy to test with an HDMI cable).
Other than that you'd probably be looking at some sort of TV input card for the PC connected to the SCART socket of the TiVo but that's not going to give a great picture (not HD) and again may be problems with copy protection etc.
Both of those will be HDMI outputs, which is why you wont get a connection.
By "all-in-one" PC, do you mean something with the monitor and PC all as one? But sounds like you're probably out of luck, there's certainly no "official" way of doing what you want so I would think that the only way you could do it would require some sort of TV input device for the PC, which would be an additional cost and probably wouldn't work especially well.
Another option might be buying a TV that both the PC and the TiVo could connect to, but if the PC already has a built in screen then obviously that's not really a neat solution.