Windows Startup Sound and Windows Logon Sound Collide

I have the following issue in Windows Vista Ultimate (Genuine MSDN
installation)

Sometimes (it happens randomly) when I start my PC, I see the vista logo
(the one before the Welcome Screen), and I can hear the Windows Startup Sound
and Windows Logon Sounds playing simultaneously. Strange thing is that
Welcome Screen doesn't appear at all. As I say, this is happening randomly
and it doesn't affect the functionality but it is annoying. Some other times,
it works as it should be, ie I see the Vista logo, hear the Windows Startup
Sound then I see the Welcome Screen and listen to the Windows Logon Sound.

I haven't installed any software apart from monthly updates and reliability
updates. Any idea why this may be happening or if there's anything I can do?
I have latest drivers for everything (chipset, gfx card, sound card)...

Thanks in advance