Virtual PC is a popular virtualization program for running different operating systems on top of Windows and Mac OS. It was first released in 1997 and has now been discontinued, partly in favour of Hyper-V.
Virtual PC was originally developed by Connectix for PowerPC Macintosh computers. Connectix Virtual PC 1.0 was released in June 1997 specifically for PowerPC based Macintosh Computers running Mac OS 8.1 or older due to compatibility issues with Mac OS 8.5 and newer. The first version of Virtual PC to run on Windows was Connectix Virtual PC 4.0. The last version of Virtual PC by Connectix was Virtual PC 5.2 for Windows.
After Connectix closed, they sold some of their products to other companies and discontinues the rest. Virtual PC and Virtual Server were sold to Microsoft Corporation, which released Microsoft Virtual PC 2004 a year later. In 2006, Virtual PC 2004 SP1 was released and Virtual PC 2007 followed next year. Service Pack 1 for 2007 came in 2008. Some betas of Virtual PC 2007 are still available online.
In 2009, Microsoft released a completely re-designed version of Virtual PC named Windows Virtual PC. The main window (VPC console) and floppy disk support were removed, but XP mode and USB support were introduced. Though designed to be incompatible with previous versions of Virtual PC, it is possible to install and use (though not simultaneously) both Microsoft Virtual PC and Windows Virtual PC on the same computer. A tutorial on how to achieve this is available here.
Virtual PC for Mac
Virtual PC for Mac has always been slow. The reason why is it has to emulate an entire x86 PC, as the PowerPC architecture cannot virtualize x86 software.. It was for this reason that Virtual PC for Mac wasn't a huge hit. It did however have some compatibility with running some programs that would only run on Windows, such as some DOS games that wouldn't natively run on a mac.
Connectix Virtual PC for Mac
Virtual PC 1.0 & 2.0 for Mac ran only on PowerPC based Macintosh computer running Mac OS 8.1 or older due to compatibility issues with Mac OS 8.5 and 8.6. One of the compatibility issues with Mac OS 8.5 is whenever you launch the program it freezes the computer. Another is that the Virtual Hard Disk won't mount when you double click on it like it does in OS 8.0\8.1.
Virtual PC 3-5 are other versions of Connectix Virtual PC. They ran on various versions of PowerPC Macs but would never run on Intel macs for there weren't any at the time and when Intel macs did come some time later Rosetta wouldn't run Virtual PC for Mac (tested on 2.26 GHz C2D iMac running OS 10.5.8).
Microsoft Virtual PC for Mac
Microsoft bought Connectix's Virtual Technology in February of 2003 they released Virtual PC version 6 for Macintosh. It would only run on Mac OS X 10.2.8+ and require at least a G3 PowerPC processor. It was still quite slow because it still had to emulate the entire x86 PC architecture in software. Microsoft also released Virtual PC 7 for Mac in 2004. It added support for PowerPC G5 Macs and added support to print from a VM without needing to install special print drivers on your host computer.