How do I find which program is using port 80 in Windows?

Start menu → Accessories → right click on "Command prompt". In the menu, click "Run as Administrator" (on Windows XP you can just run it as usual), run netstat -anb, and then look through output for your program.

BTW, Skype by default tries to use ports 80 and 443 for incoming connections.

You can also run netstat -anb >%USERPROFILE%\ports.txt followed by start %USERPROFILE%\ports.txt to open the port and process list in a text editor, where you can search for the information you want.

You can also use PowerShell to parse netstat output and present it in a better way (or process it any way you want):

$proc = @{};
Get-Process | ForEach-Object { $proc.Add($_.Id, $_) };
netstat -aon | Select-String "\s*([^\s]+)\s+([^\s]+):([^\s]+)\s+([^\s]+):([^\s]+)\s+([^\s]+)?\s+([^\s]+)" | ForEach-Object {
    $g = $_.Matches[0].Groups;
    New-Object PSObject |
        Add-Member @{ Protocol =           $g[1].Value  } -PassThru |
        Add-Member @{ LocalAddress =       $g[2].Value  } -PassThru |
        Add-Member @{ LocalPort =     [int]$g[3].Value  } -PassThru |
        Add-Member @{ RemoteAddress =      $g[4].Value  } -PassThru |
        Add-Member @{ RemotePort =         $g[5].Value  } -PassThru |
        Add-Member @{ State =              $g[6].Value  } -PassThru |
        Add-Member @{ PID =           [int]$g[7].Value  } -PassThru |
        Add-Member @{ Process = $proc[[int]$g[7].Value] } -PassThru;
#} | Format-Table Protocol,LocalAddress,LocalPort,RemoteAddress,RemotePort,State -GroupBy @{Name='Process';Expression={$p=$_.Process;@{$True=$p.ProcessName; $False=$p.MainModule.FileName}[$p.MainModule -eq $Null] + ' PID: ' + $p.Id}} -AutoSize
} | Sort-Object PID | Out-GridView

Also it does not require elevation to run.

If you want to be really fancy, download TCPView from Sysinternals:

TCPView is a Windows program that will show you detailed listings of all TCP and UDP endpoints on your system, including the local and remote addresses and state of TCP connections. On Windows Server 2008, Vista, and XP, TCPView also reports the name of the process that owns the endpoint. TCPView provides a more informative and conveniently presented subset of the Netstat program that ships with Windows.

Use NETSTAT on the command-line:

netstat util