If you mean you've got an operator customised home screen, and you want the default S.E. "4 corners" screen, then there should be an option for this.
On my Orange X10 mini, there's a pre-installed app called "Homescreen" - running that gives a choice between Orange's (awful!) homescreen, and the S.E. one
Thank you for answering but this is not the case, I mean that my operator, in this case Vodafone (I'm from Spain) has removed some official applications of Sony Ericsson, like "PlayNow" and "Sync" and I need those applications.