or is there more than one account on that machine?
Could be that occasionally Windows cannot "find" the account that you normally use and rather than just kick you off, it generates an account for you using the default settings.
Of course, this means that there is no iTunes library, or at least one with any content, available to you.
You may even notice that your desktop picture is different, assuming that you have changed the picture from the default one.
Next time it happens, right click the Start menu > Explore and check to see if there is more than one account on that machine with your name on it.
Chances are that there is.
About every 10 days or so when I boot up ITunes, it reverts to a Generic Library. In other words, it boots up as a new copy of ITunes, with my library, settings, Genius settings, etc.. gone. Fortunately, I backup my computer regularly and am able to restore my Itunes library from backup, but it is annoying to have to do this. I am running Windows 7, with ITunes version 10.