One of the cool new things in Windows Vista is the Resource Monitor. This handy new tool gives you a lot more information about what a process is doing including open connections it has to files and network addresses.
The processes page is a little different in that it gives a description for the process that is running.
Here you can now also see what services are currently running on the system and what their state is.
The performance tab has been changed quite a bit. It makes it easy to tell how much phyiscal memory you have now (note everything isn't in KB any more). The biggest difference however is the button for the Resoruce Monitor. Clicking this launches a new application which tells you very detailed information about the processes that are running.
On the CPU panel, you can see the PID, description, and threads, etc for a process. Of course you could have seen all of this with Task Manager before, but the more interesting panels are Disk and Network.
On the Disk panel, you can see what files a particular process has open as well as read and write speeds.
On the Network panel, you can see connections indivudal processes have made, how much data is transferred, and where the connection is going.
Lastly the memory panel shows you memory type things.
The Resource Manager is a really neat tool and I think will be very useful in finding out what exactly is going on with your system at a given time.