My computer goes off suddenly.

Question

my computer has windows xp sp2. for last few days there had been messages about fan speed too low . so i tried fixing the processor fan . threafter i found power supply fan too not working properly . i fixed this problem also. but the problem persisted in other way i mean one of the fans ( processor , power supply or chipset) made lot of noise while booting and computer goes off suddenly. if i try booting number of times then after two or three tries it starts but while running for a while it suddenly goes off . i watched all the three fans while booting they all are working ( not running sluggishly) so i feel cooling must not be the problem now. i want to know where the problem lies?