I had this a couple of times on a work computer. As noted, logging out and back in is usually enough, because everything you have is still in your profile* on disk, it's just that that the profile wasn't "engaged". It's like XP missed a gear change as you put the foot down.
My hypothesis is that it has to do with the services that are still loading behind the scenes as the logon screen appears. Microsoft cheats a little in this regard: when that screen appears, the computer isn't fully "up", but they judge that enough of it is going for you to log in. However, when that happens, it appears that they got that wrong, and something that's supposed to be up isn't - such as a security or virus checker.
* "Profile" has a specific meaning in the Windows world: everything under C:\Documents and Settings\<username> (XP) or C:\Users\<username> (Vista & 7). UNIX systems such as Linux or Mac OS X have the "home directory" equivalent (/home/<username>). On any multi-user system, everything that belongs to a user should be kept under the user's profile only, not scattered across the disk as used to happen in e.g. Windows 95. It's not just for security, it also makes backups more manageable, and feeds in to the argument I have with people about "backing up the computer" vs. "backing up the data".