constant application crashes

i am running a pc with win xp running on an intel p4(1.6 ghz). recently win xp has become so unstable that almost all applications (including simple ones like notepad) constantly crash within about 3 mins of launching them. most web based apps (firefox 2 & 3, ie7, opera, safari) crash immediately on launching (it took 14 attempts to connect to this site using firefox 2).

does anyone have an idea on what might be the problem? and what should i do to atleast make the system usable?

i have tried formatting my whole hard drive & reinstalling xp but it did not solve anything (so i dont think its a virus, worm, etc).