Hello, Domac and welcome to the SevenForums.
Let's begin with upgrading your Virtual PC. Uninstall Virtual PC 2007, then download and install the latest version, designed to work with Windows 7. Remember to reboot after uninstall, before installing the new version.
Remember to backup your existing virtual drives by copying all the .vhd, .vmc, .vsv and .vmcx files to a secure place.
You can download latest Virtual PC here.
Come back when you have installed the new one and we'll continue.