For Windows systems only. (Meaning, type of OS Hudson is running on)
SystemRoot and SystemDrive are needed in order for the P4 client to resolve hostnames. Why? I have no idea. Send questions to Bill.Gates@microsoft.com

If you are seeing messages in your log that look like: "Connect to server failed; check $P4PORT" and your P4PORT setting above is correct, then likely you do not these two fields set correctly.

Example:
System Drive: C:
System Root: C:\\WINDOWS