New answer, powershell

TCP

Get-Process -Id (Get-NetTCPConnection -LocalPort YourPortNumberHere).OwningProcess

UDP

Get-Process -Id (Get-NetUDPEndpoint -LocalPort YourPortNumberHere).OwningProcess

Old answer, cmd

 C:\> netstat -a -b

(Add -n to stop it trying to resolve hostnames, which will make it a lot faster.)

Note Dane's recommendation for TCPView. It looks very useful!

-a Displays all connections and listening ports.

-b Displays the executable involved in creating each connection or listening port. In some cases well-known executables host multiple independent components, and in these cases the sequence of components involved in creating the connection or listening port is displayed. In this case the executable name is in [] at the bottom, on top is the component it called, and so forth until TCP/IP was reached. Note that this option can be time-consuming and will fail unless you have sufficient permissions.

-n Displays addresses and port numbers in numerical form.

-o Displays the owning process ID associated with each connection.

Enjoy:

forfiles -p "C:\what\ever" -s -m *.* -d <number of days> -c "cmd /c del @path"

See forfiles documentation for more details.

For more goodies, refer to An A-Z Index of the Windows XP command line.

If you don't have forfiles installed on your machine, copy it from any Windows Server 2003 to your Windows XP machine at %WinDir%\system32\. This is possible since the EXE is fully compatible between Windows Server 2003 and Windows XP.

Later versions of Windows and Windows Server have it installed by default.

For Windows 7 and newer (including Windows 10):

The syntax has changed a little. Therefore the updated command is:

forfiles /p "C:\what\ever" /s /m *.* /D -<number of days> /C "cmd /c del @path"