I've written a small Visual Basic 6 Utility that will allow you to increase the size of the Live TV buffer in Windows Media Center from its default of 30 minutes up to two hours, in 15 minute increments.
Its compatible with Media Center 2005 and Windows Vista Home Premium and Ultimate. The MCE Pause Buffer can be found at:
http://www.dougknox.com or http://www.windows-now.com
It will also be available for download from http://www.thegreenbutton.com/ and http://iandixon.co.uk/cs/files/default.aspx, in the near future.
P.S. This utility may work in earlier versions of Media Center, but I have no way to test this, and don't guarantee compatility.