I have a laptop and a desktop, both running Win2kPro, connected to a D-Link DI-614+ Router. The laptop is usually connected through a WiFi card. IPs on the LAN are 192.168.0.10 and .20 and these are the only two machines here.
When using the Advanced LAN Scanner (http://www.radmin.com/lscan1b1.exe) I get the random IPs showing up as 'machines', eg:
I'm currently running a scan and have found 13 of these so far. Each of these shows as 'Not logged in' and with ports 25 (SMTP), 110 (POP3) and 80 (HTTP) open.