When you enter your password on the Linux terminal, it doesn't show anything (not letters, not asterisks, not dashes, nothing) but it does know you're typing. Hit Enter after typing your password and it will continue. If it is stuck in a loop, it's almost 100% that you're missing your password (probably typed it wrong to begin with). I would just reinstall Ubuntu altogether. Boot up an Ubuntu LiveCD and you'll be able to use GParted to edit partitions. Then you can reinstall, making sure you enter your password correctly (make sure Caps Lock is off!). Then you should be able to log in, and if you partition some empty space in, you should also have room to put XP in a dual-boot system.
A word of advice: Linux isn't the best OS for DVR/HTPC things. I have two TV cards, neither of which work well in Linux. I could never get MythTV working either, too many pieces. If you could find a copy of Windows XP Media Center Edition that'd probably be good.