How not to modify the project from Install.ps1

WrongInstall.ps1

#This script edits the project file and marks the talkfx-c.dll files to always be copied to the output directory
param($installPath, $toolsPath, $package, $project)

# Load project XML.
$doc = New-Object System.Xml.XmlDocument
$doc.Load($project.FullName)
$namespace = 'http://schemas.microsoft.com/developer/msbuild/2003'

# Find the node containing the file. The tag "Content" may be replace by "None" depending of the case, check your .csproj file.
$talkFXDlls = Select-Xml "//msb:Project/msb:ItemGroup/msb:Content[contains(@Include, 'talkfx-c.dll')]" $doc -Namespace @{msb = $namespace}

foreach($xmlNode in $talkFXDlls)
{
    $nodeName = "CopyToOutputDirectory"

    #Check if the property already exists, just in case.
    $property = $xmlNode.Node.SelectSingleNode($nodeName)
    if($property -eq $null)
    {
        $property = $doc.CreateElement($nodeName, $namespace)
        $property.AppendChild($doc.CreateTextNode("Always"))
        $xmlNode.Node.AppendChild($property)
    }
}

# Save changes.
$doc.Save($project.FullName)

Write-Warning "If Visual Studio prompts you wether to Save As... or Discard, you need to chose Discard changes."