Hello gidyn and welcome to Seven Forums.
In Windows Xp and Windows Vista you would have to add additional performance counters to Task Manager to specifically monitor disk activity. The counters are I/O Reads, I/O Writes, I/O Read Bytes, and I/O Write Bytes. I'm on a work computer running XP so I can't verify if this will work on Windows 7 but you might give it a look.
Identify Processes That Cause High Disk Activity