Inspiron 531 Not Booting & Slow Performance

Alright, this started just yesterday. When I turn on my Inspiron 531 it says "No Boot Device Selected". The hardware, though, is recognized in the BIOS. In order to boot into Windows, I have to go to the boot menu at startup and manually select my primary hard drive. After I select my hard drive, it takes a long time for Windows to actually boot to a usable state. Then, in the event log, there's over 500(!) "The driver detected a controller error on \Device\Harddisk0\DR0" errors, and they keep coming in. Chkdsk says everything's fine. Western Digital diagnostics says everything's fine. There haven't been any hardware or software changes in the last few days with the exception of the installation of a video game. System restore didn't do anything. What the heck is going on? Any help would be appreciated.

Specs:

Windows 7 Professional 64-Bit

Inspiron 531 Factory MoBo

AMD 64 X2 4400+

3GB 667 RAM

NVidia 9800 GT

Western Digital Caviar Black 640GB - Primary HDD

Seagate 250GB - Backup HDD

OCZ ModXStream 550W PSU