I had this problem ages ago... For me it was Windows firewall having a spaz...
If you are running Windows firewall:
Start>Control Panel>Windows Firewall.
On the Exceptions tab, click 'Add Port...' and enter the details:
Name: blockland
Port: 28000
TCP/UDP: TCP
Do it again, but this time click UDP, see below:
Name: blockland
Port: 28000
TCP/UDP: UDP
While you are there, check to see if Blockland.exe is on the list. If it isnt, click 'Add Program...' and search for Blockland.exe and add it.
It seemed to not matter whether windows firewall was on or off, but this fixed it.
I am not sure what to do for vista, but shouldnt be too different.
Best of luck.