Hard to diagnose without knowing your system specs, however, if it worked fine before then it might not be related to that.
Could be a corrupt driver, when you get to the boot screen (black with XP logo and blue scrolling bar) does it hang up longer than usual? because thats when the drivers are being accessed.
usually these sorts of things happen when a recent change has taken place so think back to when you last ran an updater or installed software and system restore before that date to see if that solves the problem.
Or ctrl+alt+del then go to proccess and end explorer.exe then your screen icons taskbar will dissapear. then start new task and type C:\WINDOWS\explorer.exe and it will restart explorer, its a facier way to restart everything. Might kick it into life.