My computer starts up then stops it says oc fail and i am prompted to press f1 it starts ok then?

it takes about 10 mins to start up and every thing takes 2 or 3 mins to run its now super slow . i have 512 ram and 160 gig h/drive my operating system is windows xp service pack 2 there is hardly anything on my pc so why is it slow? also ime bombarded with popups ,my antivirus keeps poping up saying it has blocked a potencial threat ,i run Norton anti virus ? please help if you can thank you