If you have too little RAM, it will still work, but everything will run slower when Windows is running. (There is an obsolete emulator, Virtual PC, which allows Windows to on some PowerPC Macs, but it is discontinued and runs slowly.). Older Macs with PowerPC processors cannot run these virtual machines. To begin, click the links in the text for each virtual machine vendor's instructions and downloads. These so-called virtual machines run Windows at the same time as Mac OS X, so you don't have to reboot. There are three third-party solutions (Parallels Desktop, VMware Fusion, and Oracle VirtualBox) which let you run a real copy of Windows on your Mac.
However, you have a few different options if you want to use Citavi on your Mac.