Computer spontaneously reboots, no BSOD, temps are ok.

This has occurred since December 2011 (when I built this computer).

The Issue: The computer randomly reboots with no BSOD. The fans and internal parts remain on, but the BIOS screen reappears and Windows restarts. The reboots occur every 15-30 minutes, every hour, or even longer. It occurs under any use: web browsing, gaming, Windows Explorer, etc. In Event Viewer, it says "Event 41 Kernel-Power" at the time when the reboots occur. The reboots occur for a few days at a time, then go away (week of Dec. 20, week of Jan. 15, and March 23-24).

• temps are ok (checked in BIOS, SpeedFan, Speccy, HWMonitor)
• no viruses detected (checked with Security Essentials and Malwarebytes)
• no overclocking

Specs:

• i5 2500k
• ASUS p8z68-v gen3 motherboard
• Corsair Vengence 8gb DDR3 RAM
• EVGA GTX 560 Ti 448 cores version
• WD Caviar Black 1tb (2 years old)
• Corsair TX 850 watts PSU (2 years old)
• ASUS Xonar Dx sound card
• Antec Nine Hundred 2 v2 case (2 years old)
• Windows 7 64 bit

Any ideas? Since the reboots occur so randomly, it's hard to swap out and test parts. Would a failing HDD cause this? I've noticed that my HDD has gotten louder over time and makes buzzing noises when accessing large amounts of data.

Thanks!

Edit: I also ran Memtest and Prime 95 for 8hrs back in Jan. No reboots occurred.