Q830786 is a fairly important  update to apply if your running Media Center. From personal expirence I've found it to greatly improve the reliability, and to a Certain extent the performance of the Media Center UI. Good new for us European users is that a few of the bugs which existed in the release build with regards to Digital Video Broadcasts (DVB-T/Freeview) were corrected. Though a number of DVB-T issues still exist.

Other major changes in this update (Build 5.1.2600.1321) are:

Terrestrial Digital Broadcasting Services

• Channel lineup editing now supports editing on a per-channel basis. When you edit services in Terrestrial Digital Broadcasting Services (DVB-T), you will only see one service next to each channel number. You will not see multiple services associated with the same channel number.
• Added support to enable two zoom modes for 4x3 DVB-T users only:
  • Zoom modes:
    • Normal
    • Zoom to fill (clipping left and right)
  • Windowed mode: The 4x3 DVB-T users will remain in the current zoom mode and continue to display the Zoom button on the More Info panel.
  • Remote codes and keyboard shortcut: The remote codes and keyboard shortcut will work for 4x3 DVB-T users.

DVR/Recording

Back to back recordings no longer cause the second recording not to be recorded.

Program Guide

• Media Center Edition now pauses for 30 seconds after it resumes from Standby before it tries to update the Program Guide. This permits computer using wireless networking cards time to reconnect to the wireless network.
• This fixes the German presets for Channel 1 so that they now work correctly.
• For users in the United Kingdom, Channel 1 now can be mapped to BBC1.

Suspend/Resume

• After resuming from suspend, you no longer see a black screen.
• This update fixes several video errors that occur after resuming from standby.

Teletext

Reveal Text will remain onscreen until it is turned off. Previously, the Reveal Text feature turned off when you viewed rolling subpages.

TV Playback

• You can now stop VCD playback without stopping any TV recording that is running in the background.
• Software installations are now not delayed when you record a TV program in the background at the same time.

Additional information sourced from support.microsoft.com