Which w3wp.exe process is which?

You have multiple w3wp.exe processes in Task Manager, and one of them is pegging the CPU. Which application pool is it?

In Task manager, processes tab, select columns and show:

"Command Line" will reveal the application pool, e.g.

c:\windows\system32\inetsrv\w3wp.exe -ap "MyWebApp" -v "v4.0" -l "webengine4.dll" -a \\, \pipe\iisipmdguid -h "c:\inetpub\temp\apppools\MyWebApp\MyWebApp.config" -w "" -m 0 -t 60

The -ap parameter is the app pool, in the example above: "MyWebApp"

A more "IIS" specific way to get this same information...

Run this command to see a map from Process ID to application pools:

%systemroot%\system32\inetsrv\appcmd list wp

If that doesn't work, try:

c:\windows\system32\cscript c:\windows\system32\iisapp.vbs