First, you need to understand that the general PCs (either desktop or laptop) uses x86-based processors manufactured by Intel or AMD. x86 is an industry standard, therefore programs made to run on x86 will run on any such processors without any restrictions.
The situation on ARM is completely different. There are no standard to adhere to, and ARM architecture can vary wildly from one ARM processor to another. Using Android as an example, it is very much possible a program that runs on one Android device won\'t work on another - one of the reasons why some apps does not show on the Android Market of different Android devices.
Windows XP, Vista and 7 does not have ARM versions. Windows CE supports a few ARM architectures.
However, given the fragmented nature of ARM, it is unlikely that CE will work on R-Pi\'s particular ARM architecture. Not to mention the drivers required for the SoC also needs to be written and made available by Microsoft.
While it\'s possible for Windows CE to on R-Pi, it\'s unlikely the relevant requirements will be met.
As pointed out in a reply below, CE does support ARMv6, the architecture in the Broadcom SoC going to be used in the first-gen R-Pi. Windows CE will work as long as relevant drivers for the SoC is written.
Now for the caveat I mentioned before - even if we do get Windows CE running, none of the gazillions of Windows software currently available will run. Nobody compiles Windows software for ARM.
If you want to run Windows because you want to run all the Windows softwares and games, well, tough luck...
Microsoft announced that Windows 8 will support ARM. But the issue of architecture and support arises again - we don\'t know which ARM architectures and SoCs will be supported.
Isn\'t it the same for Linux-based distributions?
Fortunately, no. It is true that x86 Linux and it\'s software won\'t work on ARM, but because it is open-source, everything can be easily compiled by anyone to run on ARM. And that\'s exactly what the supported distributions have been doing. The huge repository of softwares (looking at you, Debian) has been compiled to run on various ARM architectures, including the one on R-Pi. If you\'ve used Linux-based operating systems, you will find the same experience running it on R-Pi.
But that\'s just for the open-source things. Closed-sourced Linux programs like Flash won\'t be working out of the box. Same goes for the paid Linux games in Humble Bundle and other sources.