digital501 has a great review of the TiVo and Windows Media Center PCs with the objective of helping the end user choose which one is right for their needs:
There are two basic ways to enter the world of the Personal Video Recorder (PVR): through a dedicated hardware unit, like a TiVo or ReplayTV, or through a software-based PVR that runs on your home computer, like Windows: Media Center Edition (WMCE) or MythTV. In this article I will compare the most popular form of each platform, TiVo and WMCE, to help you figure out which one would be best for you.
To summarize, WMCE currently has more features than TiVo, though TiVo is catching up. WMCE requires a large cost up front, but no monthly fee, while TiVo is a lower (or no) cost up front (for basic models) but has a monthly service fee. MCPCs can also be used as home computers. Finally, TiVo is much easier to install.
Hit the link for the full details. Thomas Hawk adds to the review with details on HDTV:
Media Center can record HDTV, but only through over the air transmissions, TiVo Series 2 cannot record HDTV at all.
On the other hand, the DirecTV TiVo can not only record satellite premium HDTV (which MCE cannot) but it has four tuners (two HD, two standard def). This is the unit that I personally own and watch most of my TV on at the present time. Be careful with this unit though in the longer run as DirecTV very well could stop supporting it someday or force a trade in as they have recently begun a move from MPEG2 to MPEG4. The DirecTV HDTV TiVo supports the outgoing MPEG2.
Both Series 3 TiVo and CableLabs approved Vista Home Premium machines will record HDTV coming up later this year. Comparing a Series 3 CableCARD enabled TiVo with a Vista Home Premium CableCARD enabled PC will make for a very exciting comparison when the time is right.
More by following that link as well, including a pointer to Hawk’s earlier comparison of Media Center with the DirecTV HDTV.