Strange momentary authentication problem

I have a friend who I was visiting with and she showed me a peculiar problem she started having with her secondary laptop (an HP Pavilion DV4). It was running Vista Home Premium SP1 (not sure if it is x32 or x64). After attempting to log on, a Microsoft dialog would pop up that complained about how her computer was altered and is not validated. The dialog presented two options: to check on-line for a solution or to close. Closing it caused the computer to log off and return back to the user logon screen. Clicking on the solution option brought up the Internet Explorer browser to a Microsoft website page where you can validate your operating system. Either way, it was "wash, rinse, and repeat"--no matter what, you couldn't log on.

Well, the installed operating system came with the laptop. She said she did not install any software on it recently. It worked fine two days ago.

I did the next logical thing. I selected "Shutdown" from the bottom right corner icon. The machine shut down normally. I then booted it up. We got to the logon screen. She logged in and... TADA. Normal desktop. No errors. (If that hadn't worked, I'd have tried booting up in safe mode; ultimately I feared having to use a rescue disk, but fortunately it didn't come to that).

Well my friends, I have to say this left me baffled. Is there some manner by which Microsoft considers the current session a "genuine validated operating system" that can be momentarily corrupted? Frankly, I found it a bit unsettling. She does have Avast! Anti-virus software installed, which I've found to be a very robust product (I like it better than AVG Free). It is up to date and didn't report any intrusions or nasty things crawling about. Anyway, I promptly went to the Windows Update and got her updated to SP2. If this wasn't a "genuine" Vista installation, we wouldn't have been able to get that far and obtain SP2. Hopefully anything "fragile" that was acting up was refreshed with stable system files. Still... just curious if anybody has seen this before and knows what can cause it.

Thanks!