PC Will Boot But Only After Many Tries - Computer Freezes

My emachine PC, running on XP will, more often than not, not boot up without repeatedly pressing the start button on the tower.Before this problem began, when the computer ran normally, as soon as the ON button was pressed the cooling fans would whir at high speed for 2 seconds and then drop to a quieter, low rpm speed. The emachine logo would appear followed by the Microsoft screen, etc. Since the problem began, when the ON button is pressed the fans come up at high speed but will not drop to the quieter state, nor does the emachines logo appear. Essentially, the computer freezes.