WHEN I PLUS IN THE POWER SUPPLY TO PC, BOTH THE FANS WORK FASTER THAN NORMAL BUT THE PC WONT START?

The PC when turned on boots up both the fans but no beep or start up happens just the 2 fans, please could someone advise on what i can do? i have taken out the memory and replaced it and also i have hoovered out any dust but still no joy! the pc is only 18 months old. thanks very much any advice please mail me at cameronandy@hotmail.co.uk