Win XP SP 2 flips between classic look and XP look at star..

After installing Windows Live OneCare I observe that win XP kind of flips
oddly between windows classic view and XP view in message windows and login
window. I think the installation did something to the operating system DLL's
that manages the XP interface.
Is there a way to "tweak" or fix this behavior, so XP will behave normally
again at bootup.
I know this is odd, I have seen it on various XP machines but I'm sure there
must be a way to tune up the OS to regain the normal interface behavior.
Any help with warm thanks.