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