On a basic level only processes which are currently running will show a PID under the Services tab of Task Manager. To see and confirm this yourself, follow the instructions below.
In Windows 7, press and hold ctrl + alt + del and then choose 'start task manager' In task Manager under the Services tab, click the Status column. This will sort all services listing the 'Running' on top and 'Stopped' below. Go down to where the 'Running' and 'Stopped' processes meet, then look at the PID column and you will be able to easily see that only processes which are currently Running show a PID. Services which are Stopped will show nothing in the PID column.
It is possible to have Active Connection Protocols running which have a PID that does NOT show up under the Services tab in Task Manager. To see this for yourself follow the instructions below.
In Windows 7 go to the start menu and type cmd. A black window will pop up. Where the cursor is flashing, type netstat -ano (netstatSPACEDASHano). This will show all Active Connections on the computer. You can view PID's for protocols that are LISTENING or ESTABLISHED. ESTABLISHED PID's are actively running and connected to something... the computer, but not necessarily to the Internet or a Local Area Network(LAN).
There you may see PID's for one or more Active Connections which do not appear in the services tab of task manager at all.
Once connected to a LAN and/or the Internet, many ESTABLISHED connections may appear with various Foreign Address IP addresses shown. The PID for ALL of them may be the same, such as 6008 for example.
You will see that those Protocols are currently Active Connections but the PID may not appear anywhere in task manager; although I'm not certain why.