Use MythTV.
It's free and it works well. It won't work with newer All-in-Wonder cards.
However you can use it with the cheap Bttv-based cards, and it should work with the winPVR-250 and winPVR-350 cards.
And it kicks arse.
Ever heard of "knoppix"? It's a live CD linux. Meaning it's a OS that runs directly from the CDROM so that you don't have to install it on a harddrive.
well they made a version for MythTV
some advice
So you will be able to do something with the same functions as TiVO with out having to install anything perminately. Although you will be accessing the harddrive.
It does all the functions. Full PVR functions, record, fast foward, commercial skip, pause.
It can handle multiple cards for picture in picture type stuff, also so you can run one while recording on another card.
However it will realy shine as a full install on a dedicated Linux box.
People will do backend and frontend type things. Like have a PC box be a backend, recording everything, and have a hacked Xbox hooked up to your TV for the actual front end. The picture gets streamed over the network.
etc etc etc.
Also has some plugins and stuff for weather monitoring and video games (like nintendo emulation type stuff)
MythTV home, screenshots, and feature set
And it's free.
The trouble is that it's often not the easiest to setup. However once it gets going on a fast machine then its probably going to be one of the nicer solutions out there.