Starting from the discution on this topic: viewtopic.php?f=52&t=2293
I got an ideea.
I have just discovered that the creation of an another administrator account is not neccesary, it is enough to enable the User Account Control for the Administrator account to enable the Windows Store in the build-in Administrator account. To get there, do this: Open Local Security Policy, go to Local Policies – Security Options – User Accoumt Control: Admin Approval Mode for the Build-in Administrator Account, and click Enabled.
It seems to work fine, I have installed the default Windows 8 apps on the build-in Administrator account. No need to create a new admin account for the use of Windows Store and Windows 8 Apps anymore.