Ok this is one of the strangest issues I've run across and for that reason im thinking the problem is the power supply but need some expert opinions:

My self built computer was running fine for a good month now and i recently ordered a 8800gt to add on as I had been working off of the integrated graphics. After plugging the card in none of the peripherals would work (mouse, keyboard, nothing). I took the card out and unplugged everything except the keyboard and the keyboard lit up and worked. If I try to plug anything else in, say the mouse, it will work but the keyboard will shut off. Seems to be a power supply issue but just want to make sure it wouldnt be a motherboard problem before I drop money on a brand new power supply.

Here is what I'm working with:

Vista 64bit
250gb seagate
2gb ddr 800
ecs 8200a
Rosewill 550W
Athlon 64X2 5400BE
Arctic 64 Freezer Pro

Some other things to consider:

There has been and still is a smeared screen displayed after POST before getting to the windows LOGIN. I originally thought that it was an overclocking issue but after restoring defaults in BIOS the screen continues to come up. Not sure if this correlates or not. Also my external HD would have intermittent accessibility issues via the USB ports before this happened.

Any ideas?What is showing in device manager, any yellow marks next to a device?Nothing is yellow in there. Everything I would expect to be listed is there.OK. WELL you have to start someware and it would be with the power supply.The city you live in , do they have a Frys store?yes, not too far away.The reason i ask is when i have a problem with a unit i purchace a piece of hardware and try it out.If this is not the PBLM i return it with no PBLM. They have a 30 day return policy.I do not feel guilty in doing this because i end up buying all my hardware in the end from them.Sortware ect. Getting BACK to thePblm.By getting into windows makes me think.Without the new device added [that gave you the pblm]does the same thing happen now.Have you re loaded drivers? Re set your BIOS to best known config or default?The card that gave the problem is out and i have restored the bios to defaults. just trying to see what lead to take on hardware because I dont really see this being a driver issue since they were working before the card insert.I notice you are running Vista.Dis able the devices and turn back on the OS should recognise them & re install them. Might revert to a system RESTORE prior to when this pblm happened,[ worth a try] keep in touch.The ports are recognized, they just wont work more than one at a time. For example if i boot up with only the keyboard and try to plug in the external hd or mouse, the keyboard power goes out. If i unplug everything except the mouse, the mouse will then start working. I'll try restoring windows to a previous state though just in case.If the PBLM is still there buy a PCI/USB card and try itYa. I'll give power supply a shot and then move on to that as my second solution. Would really hate to lose accessibility to all that space though. I got my bets placed on power though, hopefully that pans out.Good luck , keep postedIn Control Panel/System/Hardware/Device Manager RIGHT clik every instance of any USB hubs and ports and select uninstall/remove.
Upon re-boot Vista will automatically re-install them...
Try this and see what happens.Figured out the problem. Seems that one small AREA of the mobo was grounded around the peripheral area which was causing the problem. A screw must have fallen out when I was installing the video card. Thanks for the help everyone!