This is as far as I know the fastest way to determine what PCs are online in your network

# nmap -sP 192.168.0.1-255

Starting Nmap 4.53 ( http://insecure.org ) at 2008-02-25 00:01 CET
Host 192.168.0.120 appears to be up.
Host 192.168.0.140 appears to be up.
Host 192.168.0.250 appears to be up.
Nmap done: 255 IP addresses (3 host up) scanned in 5.711 seconds

It only took 5.7 seconds to scan all 255 IP addresses and determine if they were online.