Very commonly posted problem, many times following new hardware / drivers
installed or updates including drivers. So when did it happen on your system?
Troubleshooting measures are Safe Mode, System Restore (to last known good
configuration) etc.
Two commonly used references to start the troubleshooting with:
Windows Driver Kit: Driver Development Tools Bug Check 0xA:
IRQL_NOT_LESS_OR_EQUAL:
http://msdn2.microsoft.com/en-gb/library/ms793589.aspx
James A. Eshelman WEB site "Troubleshooting Windows stop messages", click 0A
in left-hand column:
http://aumha.org/a/stop.htm
Good luck,
Ka2H