Can anyone help me? Apologies, I'm not super technically minded.

I ok'd a software update for iDVD & Quicktime yesterday & ever since my iMac has been doing strange things!

It's as if it's refreshing every 30 seconds. This is having different effects depending on which application I have open / are using.

Quicktime - while watching with full-screen - it minimises
Firefox - puts to the background & I have to click on the window to bring back to foreground

Mac HD - it closes the dialog box completely

Any ideas why? & how I can rectify it?