HELP A NEWBIE! Unknown Device, unsuccessful POST and SLOW windows.

Hello. I have the folowing system:

Motherboard          Intel DP55KG
Processor              i5 750 @ 2.67 Ghz
RAM                     Kingston ddr3 2 GB
Video card             Nvidia Geforce 9500 GT 1GB PCI express
Hard Drice            Hitachi HDS721032CLA367 NTSF 300GB
OS                        windows ultimate x64
Monitor                  Acer AL1916W @ 1440 * 900
BIOS Version        KGIBX10J.86A.3206.2009.0805.1855

The problem I had at first was with the unknown device USB not recognized, checked all ports, nothing was connected, tried common device manager forum suggestions. Gave up so I reinstalled Windows but I still have the same problem. And now windows visual effects are very slow, almost as if the system is on AERO slow motion, even the sound when playing music and playing solitaire. On power up the boot intel screen its kind of choppy and then  I get the following message "The system BIOS has detected unsuccessful POST attempt(s). Possible causes include recent changes to BIOS Performance options or recent hardware changes. Press 'Y' to enter BIOS Setup or press 'N' to cancel and attempt to boot with previous settings." This happens twice until windows does come up, and the windows logo starts slow. So  please help me, I you could explain it to me as if were I Child that would be helpful.