If you need to run older OSs on Windows Virtual PC you can use the AV Additions from Virtual PC 2007. This will give you VM Additions support for Windows 2000 and DOS. Get the Virtual PC 2007 install file and extract it, then run an administrative install using msiexec.exe /a <path>. In the folder you specified you will find the ISO file with the VM Additions from Virtual PC 2007. Mount the file in your guest as you would any ISO file.