Tool which finds which process is causing the heavy hard drive activity?

Resource Monitor can do this and is built into Windows 7. You can access it from a button in Task Manager's Performance tab.

Once you open Resource Monitor, expand the Disk Utilization drop-down. It will show you what processes are accessing the disk. On the Disk tab you can get more detailed information about which processes are utilizing how much disk I/O, among other stats.

As @weloytty says, the Resource monitor will help. You can also check Process Explorer from MS/systernals Process Explorer

By any chance are you running Skype on this system? If so, has it become a supernode and you are now having all that traffic? Quit Skype and be sure that the icon does not remain in the Notification Area. Even if you cannot receive calls, it can still be running

Go to Control Panel\All Control Panel Items\Performance Information and Tools\Advanced Tools and start Resource Monitor. You can see what's hitting the disk there.

http://technet.microsoft.com/en-us/edge/Video/ff710685

I FOUND IT! It's a stupid piece of software called Seagate Dashboard, and it automatically installs itself in your 'puter when you plug in a Seagate external hard drive. For some reason, this POS does something cyclically, about once a second. Whatever it's doing involves RAM, and as usual Windows -- in combination with anti-virus software, which for some reason uses a dreadful lot of RAM -- has all the RAM occupied, so the machine has to resort to virtual RAM and starts driving the HD. Getting the virtual RAM adjusted takes about a second -- just long enough for Seagate Dashboard to hit it AGAIN. The system is constantly occupied with this cyclic RAM fiddling, which doesn't show up as an activity (!) and prevents anything else from working right.

Fix is simple:

Start -> Control Panel -> Administrative Tools

double-click on Services

scroll down the list to find Seagate Dashboard Service

right-click on it and hit "Stop". Things will get a LOT better immediately!

right-click on it again and change "automatic" to "manual" -- or perhaps even "disable" -- so it won't bother you any more.