Never underestimate the ability of Microsoft Windows to throw cryptic error messages (I look forward to the new delights of obfuscation that Vista will bring!). When booting my older PC yesterday, I was greeted with this pearler:

A problem is preventing Windows from accurately checking the licence for this computer. Error code 0x80090019

There are, it seems, a bunch of different things that can cause this problem, all of them related to Windows’ (in)ability to load the default cryptography DLLs with which it checks your registration details (every time you boot, apparently). One cause mentioned by the MS Knowledge Base article is your drive letters getting stuffed up, and another is if HKEY_USERS\.DEFAULT\Software\Microsoft\Cryptography is somehow damaged. But my drive letters hadn’t changed and I didn’t even have that registry key to begin with.

The simplest of the other suggestions that I could find was to boot Safe Mode and run System Restore — which I, in my infinite foresight, had turned off. Some folks spoke of running Windows Product Activation one more time, and the last-ditch fix is to reinstall completely. Obviously those weren’t particularly palatable options.

But then this article made me realise that the problem might, in fact, lie in HKEY_LOCAL_MACHINE\Software\Microsoft\Cryptography. So I took a known working copy of that key from my other PC, and applied it to the broken one in Safe Mode. My suspicion of borkedness was promptly confirmed — after a reboot, everything was fine.