[eschwartz]

When calibre version 2.0.0 was released, it was transitioned over to a new graphical toolkit, Qt5. Qt no longer supports Windows XP (and neither does Microsoft, so there is little hope of anything changing).

As a result, the latest version of calibre to work on Windows XP is calibre 1.48. If you want to run calibre on Windows XP, you have a few options:

1. Install version 1.48 (download it HERE) -- yes it is an old version, but it still works just fine, doesn't it. It hasn't magically stopped doing what it used to do just because there is a newer version.
   This is the official recommendation, as suggested on the calibre website.
2. Insist on running the latest version anyway. Yeah, Qt isn't supported. Yeah, it might crash horribly. But it *might* work, people *have* reported success. As long as your processor supports the SSE2 instruction set, you may very well get lucky.
   You must run from source -- the calibre developers will not support this.
   • Follow the directions here to set up a calibre development environment: http://manual.calibre-ebook.com/develop.html
   • Download the latest version of calibre. It won't install, however, you can extract the Calibre2 folder using lessmsi (has GUI) or directly via the Windows Installer Service. Delete the corresponding folder "C:\Program Files\Calibre2\" and replace it with the one you extracted.
   • Go to your calibre source checkout. Edit the file "src/calibre/gui2/__init__.py" and change the line that says "if isxp:" to "if False:" -- this disables the incompatibility error.
   calibre 2.x will now run -- whether it works remains to be seen. Good luck.
3. Trying to run the new version is too risky, but the old version is broken!
   Do you have a Kobo H2O, the driver for which appeared post-calibre2.x? If so, try this plugin, or run from source as described above, but with my backports described HERE. The backports can and will work to fix other breakage, e.g. Amazon website changes for metadata download. The plugin is just for the Kobo.
4. Upgrade your computer. Microsoft no longer supports Windows XP, not even for security vulnerabilities. Okay, they did anyway. But we can safely assume it won't happen again.
   • For a few hundred dollars you can get a brand-new computer that runs Windows 7/8 and does everything you need it to do, with up-to-date software -- that includes running the latest version of calibre.
   • I don't have the money/I don't want to spend the money on a new computer, my old one works fine no matter what Micro$oft thinks!
     Rather than upgrading the hardware, maybe you can upgrade the OSware? Linux is free, and runs fine on old hardware -- even if your computer isn't compatible with Windows 7/8, you can still run a modern operating system. You can even run both linux and Windows XP on the same computer.
     Popular choices of linux distribution for linux newbies include Ubuntu and Linux Mint. I suggest the latter -- it has a friendlier desktop and is closer to the experience one would expect coming from a Windows environment.
     Installing linux is fairly easy -- all you need to do is burn a disk image to a USB drive and reboot your computer -- and be good at following (clear) directions.