Norton warning: fake or real?

Hi all,

I received the following warning from Norton Internet Security (translated, original is in Dutch). My question is: is this warning real of fake? I was not even trying to install a new version of Norton...?

"Warning! A problem requires your attention: Norton Internet Security can not be installed since an other version of Norton Internet Security is already installed. Remove the current version, restart the computer and re-execute this installation programme." 

However, as mentioned before, I was not even trying to install a new version? So, what do I do with this warning?
All help is greatly appreciated!

Regards,

Almar Snippe