Windows XP interrupt request levels:

31: HIGH_LEVEL
30: POWER_LEVEL
29: IPI_LEVEL
28: CLOCK2_LEVEL, CLOCK1_LEVEL
27: PROFILE_LEVEL
… Device interrupt levels
2: DISPATCH_LEVEL
1: APC_LEVEL
0: PASSIVE_LEVEL

Controlling IRQL:

KIRQL oldirql;
ASSERT(KeGetCurrentIrql() <= DISPATCH_LEVEL);
KeRaiseIrql(DISPATCH_LEVEL, &oldirql);
….
KeLowerIrql(oldirql);