Problems After Upgrade to 3.1.3

I have just upgraded one of my blogs to 3.1.3, and now I get the same error whenever I attempt to do anything: Write a post, create a new category, delete a plugin. Its the same error, just to different file locations.

The error reads – Warning: Cannot modify header information – headers already sent by (output started at /….etc

I have read previous posts re ‘whitespace error’ – however this seems to be a global issue across every function.

Any help would be appreciated.