hey everyone, im new to this forum, and i really need some help with my PC, its very strange thing..

I turn my computer on, and it kind of jsut turns on but doesnt start up,
The cd drives flash a fair bit, but stillno start up, the monitor stays on stand-by,
and nothing happens for about 10 to 20 minutes, then suddenly it will start up, screen turns on
and start up beings as normal, with all the beeps and stuff as usual, it is very irritating when i need somehting off the internet quickly, and i have to wait 20 minutes or any given time really, till my computer will even turn on correctly.. can anyone help me out here please !!!