Now it is mandatory to install the same * application to a windows XP computer without the service pack 2 installed and it seems that I can't install that service pack there.
I'd try to convince the customer to upgrade XP to service pack 3. MS has made a serious effort when it comes to improve the security of their OS. If those aren't installed your system will be open to what is now well known flaws that can easily be exploited.
Have a look at: Security: It’s Getting Worse
The practice of never patching production systems makes automation systems particularly vulnerable. So you will be doing your customer a favor, and save yourself a bit of trouble.