It seems that the recent release of Apple's iTunes 7.4 has caused some major problems for systems running it in a 64bit Windows environment as it's no longer supported. This is a problem I'm experiencing first hand with the dreaded "iTunes was not properly installed" message. Apple have an article titled iTunes for Windows not currently supported on any 64-bit editions of Windows in their support area detailing how users can check if their operating system is 64 bit. Now it's a shame they've not said why support has been removed, or when/if they're going to fix it.
I also wouldn't be surprised if iTunes running in Leopard still supports 64-bit processors - almost seems like a cunning ploy to turn people away from Windows don't you think?
UPDATE: The new release 7.4.3.1 that patches problems with the touch based iPods still has this problem.
go to http://www.gearsoftware.com/support/drivers.cfm and download the proper version for your setup (the top one for 32bit versions, the middle for 64bit ones).
Here's the direct link: ftp://downloads.gearsoftware.com/upd...MD64_EM64T.exe
that should do it, when you look in the preferences under itunes, go to the "advanced" tab and click the "burning" tab. you should see you're burner listed there and itunes will stop giving you that warning message.