Terry Marshall asked if he could move his old hard drive, unchanged, to his new home-built PC, boot his existing, "very stable" XP installation, and skip Windows 7 altogether.
The short answer is no. Here's why:
Windows is a very hardware-specific operating system. When you install it onto a computer, it configures itself for the hardware. Unless the two PCs are physically identical, taking the hard drive out of one and using it to boot another is asking for trouble. The result will be very unstable--if it runs at all.
If your computer didn't come with Windows pre-installed, you must install the OS from scratch. And then you have to install your applications and move over your data.
There's another issue: Microsoft puts limits on what you can do with a single copy of Windows. If it suddenly appears to be running on new hardware, it will object.
Does that mean you have to give up your beloved XP? Not necessarily. If you installed a full, retail copy of XP onto your old computer, you can use the same CD to install it onto your new one. The installation program will probably object when you enter the license number, because the copy is on another computer, but you can call the provided 800 number and explain the situation.
As long as you promise to remove XP from the old computer in the near future, and keep that promise, there should be no trouble.
But if XP came with the old PC, the license is strictly for that computer. You can't transfer it. You can, however, still buy XP--even if Microsoft is no longer selling it. A Google Shopping or Pricegrabber search will bring you plenty of choices.
Contributing Editor Lincoln Spector writes about technology and cinema. Email your tech questions to him at firstname.lastname@example.org, or post them to a community of helpful folks on the PCW Answer Line forum. Follow Lincoln on Twitter.