The BAD_POOL_HEADER blue screen can occur when Windows 7 fails to allocate memory correctly when a pool header is corrupt. As with most errors, there are many possible causes - so you will need to perform some diagnostics to narrow down the problem.
Common causes of this error are:
Bugs in system drivers
The first step in troubleshooting should be to update all of your system drivers, especially those used for your chipset and SATA controllers (as these are often the most frequent culprits). If you are unsure which chipset drivers you will need, please post your system specs on the forums and we can point you in the right direction. This page also has links to the common chipset manufacturers, all of which have Windows 7 drivers. You should also make sure that your system is fully updated with the latest service packs (via Windows Update).
Should this BSOD error persist, you can revert your Windows installation back to an earlier point in time using System Restore. Booting in to safe mode should prompt for this option once you are at the desktop.
You can also use a crash dump to find more details on the error, allowing you to pinpoint any problematic files. The dump files are located in C:\Windows\MiniDump and a detailed article explaining how to analyse these will be created soon.