USB devices aren't recognized

I have a Dell Studio which is about 6 years old. It came initially with Windows 7 and I'm running Windows 7 Ultimate right now. About four days ago my computer stopped recognizing my usb mouse. I would have it plugged in and using it, walk away for a minute, and come back and it wouldn't work. I unplugged and replugged it in a few times and it worked. Then it did it again and wouldn't work no matter what I did, in any of the ports. I tried multiple other mice and usb devices and the computer won't recognize any of them. Finally, I reinstalled windows 7, thinking there was a problem with the registry or something, and it started working again. I went through the hours of updating and reinstalling some of my stuff then this morning the same thing is happening again. It won't recognize devices.

I went to Dell website and updated all my drivers, my bios and chipsets. I've restarted repeatedly. I've checked the USB section on device management and tried to find any problem area but there is none. I've run dell diagnostic and windows diagnostic and all say they can't figure out the problem. I'm stuck. What am I supposed to do? I've tried unplugging the power supply, removing the battery, doing basically everything that came to mind. I really need some help and ideas on how I could fix this. It's really frustrating me. I wiped my computer and went through all those hours of work just for it to mess up again today and I'm very annoyed. Thanks in advance for any help.