Try out a memory stick in the USB port, if it doesn't work then I would say your Power Supply is faulty or you need a bigger one. Re later only if you have upgraded say a graphics card.
is your machine a branded make and if yes what make and model like Dell with Service Tag number.
Have you installed anything new recently to just before this problem started happening maybe. Check all your drivers in device manager. Look for Yellow Exclamation marks and post back with any problems their.
in Device manager, right click on Disk Drives and note down your HDD model Number listed, maybe a HDD test may be a good idea.