Windows – Tool which finds which process is causing the heavy hard drive activity

It's not clear what you're asking. You ask about "total process memory" as if that were a well-defined term. It's really not. The types of memory measurements that seem natural to people who don't have a deep understanding of memory management basically don't exist on modern systems.

A more useful way to do this is for you to tell us what it is you are trying to figure out. We can tell you how to do that. People tend to say things like "I need to know how much memory a process is using", but that's largely meaningless. It's like asking how much space a person is using. Well, they share the bathrooms, should that count? And they have some stuff stored in the garage, but only because nobody is using it and they could throw that stuff away if anyone needed the space, should that count?

There's really no substitute for studying in some level of detail how modern operating systems manage memory, the distinction between physical and virtual memory, clean versus dirty pages, and so on. That way, you can make sense of the numbers the system actually can provide you.

System is the Windows kernel. It is not very observable from userspace. This is often caused by:

• Faulty hardware
• A misbehaving driver
• Very inefficient disk storage subsystem stuff (e.g., encrypted software RAID-5)
• A rootkit

The approach to beginning to solve this is incredibly specific to your situation, but in your shoes I'd try a few random things like:

• Running a SMART test on the hard drive
• Booting another operating system using e.g. a Live CD of Linux (Ubuntu, for example) and check out the I/O performance in there to rule out any software or driver issues
• Reinstalling the OS if it does appear to be a software problem
• Replacing hardware if the performance is the same even using another OS