Media Centre Jumps At Menu And During Playback

I own a laptop which came with WMC and Vista pre-loaded. Playback has been
superb up until recently when the mouse and loading screen began stopping and
starting on the scrolling menu screen. I disabled transitions and sound
effects in the settings which only slightly improved the jerkiness. The
problem is in DVD playback as well; the audio is fine but the visual will
stop and start to catch up with the audio. I have the same drivers that have
worked all the time and haven't installed any programs recently. My Windows
Media Player plays without the jumps (although it does have 'copyright
protection issues-another issue!), so this problem is localised to just the
Windows Media Player.