getting strange error - cannot modify header information

I have installed drupal and been running it for three weeks with no problem. This morning though when ever any page loads up I get this error:

"warning: Cannot modify header information - headers already sent by (output started at /htdocs/www/index.php:2) in /htdocs/www/includes/common.inc on line 148."

Now when some presses submit for anything a white page loads up. Content will load up, but people cannot use things such as the search box that returns data after they make an entry and if they make a post the post gets saved but a blank pages comes up afterwards.

I have tried different themes and that hasn't made a different. disabled my modules and that hasn't helped either. Any ideas?

hmm - I have another dupral site on the same sever that haver been running for years and it is giving the same error two... is there a change on the server that could cause this?