Hmm well as a "newbie" at computer assembly and stuff...can't say that building a tivo is exactly the best project to pick first. However, if you're ready to dive in head first, pick up a Hauppauge capture/tuner card and check out
www.mythtv.org. The "manual" installation is probably going to be a bit much, but there are some pre-built distros that you might try (see the web links page).
If you would rather go the Windows route, you can always just get an ATi All-in-wonder video card and install the PVR software that comes with it. From my experience the ATi PVR stuff is kinda lame though. MythTV looks freaking AWESOME and I'm planning on setting up a MythTV box as soon as I can collect all the parts.