Delayed startup after switching on computer

Hello i was wondering if someone could help this is my problem:

I switch on my computer using the power button on the tower; once i have done this nothing happens for around two minutes no start up lights or any sounds. After around 2 minutes the pc will all of a sudden just start up as normal. (Sorry this is vague, however im not looking for a very specific response to my problem just some general ideas as to what maybe causing the problem)

Thanks for any help.