Please visit this link: Asus tek computer inc. -support- drivers and download p7p55d le
ASUSTeK Computer Inc. -Support- Drivers and Download P7P55D LE
ASUSTeK Computer Inc. - Motherboards- ASUS P5K-VM
Scroll down to the utilities category, then scroll down to the "atk0110 driver for windowsxp/vista/windows 7 32&64-bit" (it's about the 12th item down).
Download and install it.
Go to c:\windows\system32\drivers to check and make sure that the asacpi.sys file is date stamped from 2009 or 2010 (notbefore).