This happens to me all the time, too. I can't find any pattern to it.

-=-=-=-=-=-=- System Information -=-=-=-=-=-=-

SYSTEM_OS Windows 7 Ultimate 64-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_gdr.130708-1532)
SYSTEM_CPU Intel(R) Core(TM)2 CPU 6400 @ 2.13GHz (2 CPUs), ~2.1GHz
SYSTEM_CORE_NUM SYSTEM_CORE_LOGIC 2 SYSTEM_CORE_PHYSICS 2
SYSTEM_BIOS Phoenix - AwardBIOS v6.00PG
SYSTEM_LANGUAGE English (Regional Setting: English)
SYSTEM_MEMORY 4094.488MB
SYSTEM_VIRTUAL_MEMORY 2047.875MB
SYSTEM_PAGE_FILE 8187.160MB
SYSTEM_STORAGE C:\ SYSTEM_HARDDISK_DRIVE SYSTEM_TOTALDISC_SPACE 238.375GB SYSTEM_FREEDISC_SPACE 163.602GB
SYSTEM_STORAGE D:\ SYSTEM_HARDDISK_DRIVE SYSTEM_TOTALDISC_SPACE 931.511GB SYSTEM_FREEDISC_SPACE 707.200GB
SYSTEM_DIRECTX_VERSION DirectX 11
SYSTEM_GRAPHICS NVIDIA GeForce 8800 GT
SYSTEM_GRAPHICS_DEVICE \\.\DISPLAY1
SYSTEM_GRAPHICS_CHIP GeForce 8800 GT
SYSTEM_GRAPHICS_MAKER NVIDIA
SYSTEM_GRAPHICS_VRAM 495.563 MB
SYSTEM_GRAPHICS_SHARED_VRAM 1791.244 MB
SYSTEM_GRAPHICS_DAC Integrated RAMDAC
SYSTEM_GRAPHICS_DISPLAY_MODE 1920 x 1080 (32 bit) (60Hz)
SYSTEM_GRAPHICS_DRIVER nvd3dumx.dll,nvwgf2umx.dll,nvwgf2umx.dll,nvd3dum,nvwgf2um,nvwgf2um
SYSTEM_GRAPHICS_DRIVER_VERSION 9.18.0013.1106
SYSTEM_GRAPHICS_DRIVER_DATE 2/26/2013 1:32:38 AM
SYSTEM_GRAPHICS_DRIVER_LANGUAGE English
3.0
SYSTEM_GRAPHICS_PIXEL_SHADER 3.0
SYSTEM_GRAPHICS_VERTEX_TEXTURE SYSTEM_SUPPORT
SYSTEM_SOUND Speakers (Creative SB Audigy 4 (WDM))
SYSTEM_SOUND_DEVICE_ID {09FAFBBC-617C-4468-B185-9F638350C9E7}
SYSTEM_SOUND_DEVICE_NAME ctaud2k.sys
SYSTEM_SOUND_DRIVER_VERSION 6.00.0001.1377
SYSTEM_SOUND_DRIVER_LANGUAGE English
SYSTEM_SOUND_DRIVER_DATE 3/18/2010 8:50:52 PM
SYSTEM_SOUND SPDIF Out (Creative SB Audigy 4 (WDM))
SYSTEM_SOUND_DEVICE_ID {89AB5C7C-AF81-4FD3-92E5-6908A4C0C62E}
SYSTEM_SOUND_DEVICE_NAME ctaud2k.sys
SYSTEM_SOUND_DRIVER_VERSION 6.00.0001.1377
SYSTEM_SOUND_DRIVER_LANGUAGE English
SYSTEM_SOUND_DRIVER_DATE 3/18/2010 8:50:52 PM
SYSTEM_INPUT {6F1D2B60-D5A0-11CF-BFC7444553540000}
SYSTEM_INPUT_NAME Mouse
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 3 SYSTEM_GAMEPAD_BUTTON 5 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {6F1D2B61-D5A0-11CF-BFC7444553540000}
SYSTEM_INPUT_NAME Keyboard
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 128 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {CD637CD0-E3C4-11E1-8001444553540000}
SYSTEM_INPUT_NAME USB Keyboard
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 1 SYSTEM_GAMEPAD_BUTTON 1 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {CD652A80-E3C4-11E1-8002444553540000}
SYSTEM_INPUT_NAME USB Keyboard
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 3 SYSTEM_GAMEPAD_FOV 0
SYSTEM_INPUT {CD652A80-E3C4-11E1-8003444553540000}
SYSTEM_INPUT_NAME USB Keyboard
SYSTEM_INPUT_REMARKS SYSTEM_GAMEPAD_AXIS 0 SYSTEM_GAMEPAD_BUTTON 21 SYSTEM_GAMEPAD_FOV 0