This is usually a firewall or windows update issue.
First try to reboot.
If no help, try this:
Start > Programs > PostgreSQL > 8.x > Stop Service
Start > Programs > PostgreSQL > 8.x > Start Service
If that doesnt help delete the \postgresql\8.x\data\postmaster.pid file, if you see it. And reboot.
Add exceptions to the windows firewall, even if it is off. If you have a 3rd party firewall,
If that doesn't help you will have to reinstall PostgreSQL completely postgresl.org