I have written a kernel mode driver using Jungo's WinDriver. At the moment I have a PCI card generating interrupts every 1ms. I deal with the interrupts within the Interrupt Service Routine and take time stamps (from the PCI card) when the ISR is entered and exited. The problem I have is that seemingly randomly the time taken to complete the ISR is almost 1ms. Although every interrupt is captured and dealt with I would like this to be decreased (most of the interrupts are dealt with in 2.5 microsec). Can anyone tell me what is interrupting my ISR (I didn't think this was possible) and is there anything I can do to stop it?