I5, Z68, Trashing Disks, No Load, No Activity

I'm having this weird problem with a PC trashing disk to the point where the PC becomes completely unresponsive, yet, perfmon does not seem to indicate any load. CPU is 1-3%, memory used <1.5GB. Disk-IO is indicated to <100 KB/sec, yet, and this is the weird bit, the blue line "highest active time" hammers on at 100% non-stop, and the HD activity light is on constant.

This is the hardware:

GIGABYTE Z68MA-D2H-B3 MB
i5-2400 CPU
2 x 4GB 1333MHz RAM
2 x 320GB WD SATA3 (3GB) disks connected to the MB 3GB SATA connectors.

It started with an upgrade from an AMD64 with an Asrock MB which had worked fine for years. First I just replaced the MB, CPU and RAM and after the usual complaints by MS, Windows 7 eventually settled and everything worked fine for a couple of days until suddenly the trashing that I described above started. I thought this was perhaps due to some leftover drivers from the AMD time, so I reformatted and reinstalled Windows 7. The trashing eventually goes away, for example if I leave it alone overnight, it might have calmed down in the morning, or if I turn off the PC and turn it on again, it may take anything from a couple of minutes to a couple of hours, but eventually it starts again.I already checked for malware. I've used Avast for years, but decided to switch to MSE to see if it made any difference (it didn't). I also ran Malwarebytes, also didn't find any problems. Windows 7 has been updated with all the latest updates and patches and SP1 as well as with the supplied Gigabyte Drivers.