Prerequisite: Windows 7 Administrator rights
The following steps are required to change the number of cores a program can access:
- First press Strg Alt Entf to open the Task Manager
- Then select the Process Tab
- Right Click the corresponding process of the Program(ie. Firefox firefox.exe)
- Choose Processor Affinity and choose the cores
It seems that the actual versions of xampp are for 32bit-versions of Windows operating systems. But after some search in the internet I found the good hint that all the parts of the xampp package are also alone available and I just needed the webserver (apache), php and mysql and all three are available in a 64 bit version.
I've downloaded that three that are normally part of the package, installed them make some changes in the configuration (I found a tutorial on the internet for this) and it worked.