You sure it's not antivirus software running checking all HDD and network activity ?
AV wouldnt trigger hardware interrupts, as the hard drive should be running in DMA mode. If its PIO then all disk access would trigger hardware interrupts, and in that case AV software would create extreme cpu load. Most network cards use DMA transfers as well, so shouldnt affect cpu load too much. The scanning itself would generate real cpu load rather than hardware interrupts.