My HP is quite old, but at the time it came with a Sonic program that enabled burning of CD/DVDs as well as WinDVD. You will have to see what software is included with yours. Of course, you can always uninstall the software you do not want via control panel. But, some people are adamant about installing a fresh copy of Windows only (which can only be done with a separate full license from a Microsoft packaged product license).
There is an "Anytime Upgrade" (http://www.microsoft.com/windows/windows-vista/get/anytime-upgrade-overview.aspx) that you can do to Ultimate. It costs $159 from Home Premium to Ultimate. If you already own a license to Ultimate, I believe there is also a way that you can upgrade via the Anytime functionality, but I am not 100% on that. Using this upgrade, you should keep all of your files/settings/etc...
Tru2Way does not really exist yet, so there is no guarantee about upgrading to those. There would also need to be a Windows/MCE upgrade as well to enable that functionality.
The COA is very important. It is not replaceable since it is supposed to be on the side of the box...same goes for your Windows license key. The Cablecard COA is very hard to get as you can only get 1 with a new cablecard PC from an OEM. In addition there is a special BIOS in your computer that has additional cablecard bits. This is all part of the ridiculous requirements from CableLabs (controlled by the cable TV industry), which really do not want you to do anything other than rent a box from them.
1: Dimension 9200 - Vista Ult, 2.4GHz Q6600 Core2Quad, 4GB RAM, 160GB, 750GB, 500GB USB, Nvidia 8300GS, PVR-150, HDTV Wonder, Vbox 3560, X360, DMA2100
2: HP m376n - MCE 2005, 2.6 P4 w/HT, 1GB RAM, 250GB, 400GB USB HD, Nvidia 6200, Emuzed MAUI, X360