ntoskrnl.exe may be corrupt - replace the file as directed
Eliminate unnecessary software you've installed since owning the system.
Manually look at manufacturer's sites for drivers - and Device Maker's sites.
http://pcsupport.about.com/od/driverssupport/ht/driverdlmfgr.htm
Installing and updating drivers in 7 (updating drivers manually using the methods above is preferred
to ensure the latest drivers from System maker and Device makers are found)
http://www.sevenforums.com/tutorials/43216-installing-updating-drivers-7-a.html
Stop Windows 7 from Automatically Installing Device Drivers
http://helpdeskgeek.com/windows-7/stop-windows-7-from-automatically-installing-device-drivers/
Disable Windows Update Device Driver Search Prompt In Windows 7 / Vista (for Professional, Ultimate, and Enterprise)
http://www.addictivetips.com/windows-tips/disable-windows-update-device-driver-search-prompt/
Most likely reason for blue screen errors is outdated/corrupt drivers.
Visit the manufacturer’s website and download and install the latest Windows 7 compatible drivers.
For more information on updating drivers, see Update drivers: recommended links
I’d recommend you to follow all the methods from this article to troubleshoot blue screen errors:
Resolving stop (blue screen) errors in Windows 7