My computer just did something really weird?

ok, so i have a windows XP desktop (im on my laptop now) and i was just using it, and i was on the kelly clarkson website and i tried to go to facebook and google but neithor of them would go (and it wasnt frozen up) so i hit the X on to close the window and something popped up that windows was busy and if i closed the window it might cause some problems, so i just turned the whole computer off from the power strip its plugged into (i do that alot due to it constantly freezes) and this weird thing came up that it was like checking my C: disk to see if everything was like accurate or something (i've reset it like 100s of times and its never done that) and then when it finished it just turned on and went back to normal, my question is what the hell just happened and why did it do that?