Need more info:
Where are you connecting from? Same LAN or somewhere on the Internet?
If you're connecting from the Internet, how are you reaching a PC on your LAN? Do you have a Static IP from your ISP? Are you using a Dynamic DNS account? What type of Internet connection are you using to connect remotely (cable, dsl, wireless, etc)?
How is your network built? Who is your ISP? Do you use a Router or does your PC connect directly to a cable/dsl modem? Are there other PCs or devices on your network?