During Windows XP startup, why do I sometimes have to press Enter or Esc at the welcome screen?

Sometimes during boot-up (there's no pattern that I can make out) Windows XP will get stuck at the blue 'Welcome' screen. If I press Esc or Enter then Windows continues to boot up and everything is fine, any other key and it just sits there. It's like there's a dialogue box hidden behind the blue.

Has anyone encountered this before or knows what it might be?

Edit: Just been thinking again about ChrisF's reply. While I don't have any thumbdrives, etc attached I do use a Microsoft Wireless Keyboard and Mouse. This thing loves to tell me that its batteries are running low (sometimes as early as five minutes after a replacement) by popping up a dialogue box. Is this likely to be the cause?

Weird. When computers are doing weird stuff I tend to scan them with malwarebytes as a matter of course. Could be worth looking in the registry for what's running at startup to see if there's anything odd there?

Have you tried changing to logon method to entering username and password rather than clicking on the profile you want?

What external devices do you have attached? There might be an error message coming from one of those. Before shutting down safely remove any thumb-drives, mp3 players etc and re-boot.

As you say your problem only occurs sometimes, you might have to do this several times. Unfortunately as you're trying to prove a negative you'll only know it's not the thumb-drive (or what ever) when the problem occurs with that device removed.