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Microsoft Virtual PC is a virtualization suite for Microsoft Windows operating systems, and an emulation suite for Mac OS X. The software was originally written by Connectix, and was subsequently acquired by Microsoft. In July 2006 Microsoft released the Windows-hosted version as a free product. In August 2006 Microsoft announced the Macintosh-hosted version would not be ported to Intel-based Macintoshes, effectively discontinuing the product as PowerPC-based Macintoshes are no longer manufactured.
Virtual PC emulates a standard PC and its associated hardware. Thus, it can be used to run nearly all operating systems available for the PC. However, issues can arise when trying to install uncommon operating systems that have not been specifically targeted in the development of Virtual PC.
Virtual PC emulates an Intel Pentium 4 processor (but virtualizes the host processor on Windows versions) with an Intel 440BX chipset, a standard SVGA VESA graphics card (S3 Trio 64 PCI with 4 MByte Video RAM, adjustable in later versions), a system BIOS from American Megatrends (AMI), a Creative Labs Sound Blaster 16 PnP (native Vista audio when Vista acts as host and guest), and a DEC 21041 (DEC 21140 in newer versions) Ethernet network card.
Not all programs are guaranteed to work because they can use undocumented features of hardware, exotic timings, or unsupported opcodes, although overall compatibility is considered excellent. Nonetheless, there are many issues that remain which detract from the overall experience, even within the expected/targeted OSes. For example, a bug in VirtualPC 2004 SP1 in Windows is such that if there exists any installed printer using LPT1 in the host OS (i.e. Windows XP), the guest OS will be unable to use the printer port at all. This problem was corrected in Virtual PC 2007.
It also uses some guest calls traps (especially when using the guest extensions) to accelerate emulation or offer additional features, such as integration with the host environment.
The first version of Virtual PC was developed for the Macintosh and was released in June 1997. Four years later in June 2001, the first version of Virtual PC for Windows, version 4.0, was released. Connectix sold versions of Virtual PC bundled with a variety of operating systems, including many versions of Windows, OS/2, and Red Hat Linux. As it became clear that virtualization was important to the enterprise, Microsoft became interested in the sector and chose to acquire Virtual PC and an (at the time) unreleased product called "Virtual Server" from Connectix in February 2003.
Version 5.0 was ported by a third party for running with OS/2 as host. This version also included guest extensions for OS/2 guest, that can be used with Windows, OS/2 or Mac OS X hosts using Virtual PC versions 5, 6 or 7. A new version of the guest extensions was included with Virtual PC 2004.
On July 12, 2006, Microsoft released Virtual PC 2004 for Windows as a free product, though the Mac version must still be purchased. The Windows version may be downloaded from here. The equivalent version for Mac, version 7, ends up being the final version of Virtual PC to be released for Mac.
On October 11, 2006, Microsoft made available the first public beta release of Virtual PC 2007 to testers. As expected, Virtual PC 2007 is only available for the Windows platform. The main improvements over Virtual PC 2004 are support for hardware virtualization and for Windows Vista as both host and guest (although Vista guests currently cannot make use of Aero Glass due to the limitations of the emulated video hardware). Virtual PC 2007 hosts can also run on 64-bit versions of Windows although 64-bit guests are yet to be supported. Users who wish to participate in the beta program can enroll through the Microsoft Connect website.
Also on the Microsoft Connect website is the beta of Virtual Machine Additions for Linux.
On February 19, 2007, Microsoft released the final version of Virtual PC 2007.
Microsoft announced on August 7, 2006 that Virtual PC for Mac would not be ported to the Intel Mac platform. Microsoft stated “alternative solutions offered by Apple and other vendors, combined with a fully packaged retail copy of Windows, will satisfy this need.” 
Apple had previously announced and shipped a preview release of Boot Camp. According to Apple, a finished version of Boot Camp will ship with Mac OS X 10.5 "Leopard." Boot Camp will allow Windows XP to be installed on different partition of the hard drive of new Intel-based macs. This allows a user to boot into either Mac OS X or Windows, though not simultaneously. The inclusion of a solution to running Windows based-applications on Macintosh hardware decreases the need for an Intel version of Virtual PC.
There is also a competing product from Parallels, Inc. called Parallels Desktop for Mac. This application is designed to take advantage of Intel's new technology called Intel Virtualization Technology which is part of the new Intel Core processor which the majority of Apple's latest computers use. By using I-VT, it increases the performance of the guest operating system, making it more practical for regular use. Parallels Desktop also emulates newer hardware than Microsoft Virtual PC. VMware and VirtualBox have also announced that they will be releasing a Mac OS X version of their software.