PowerShell Script that changes all JavaScript- and CSS-Links in several files

To achieve this, you'll need to do some kind of search and replace, the following should help you on your way, this uses a guid for the unique identifier.

$guid    = [guid]::NewGuid()
$Search  = "myscript.js"
$Replace = "myscript.js?v=$guid"

Get-ChildItem -Path "C:\inetpub\wwwroot\Inet\MyApp\Views" | ForEach-Object {
    get-content $_ | % {$_ -replace $Search,$Replace} | Set-Content $_ -Force
}

One thing to mention though is that MVC 4 can do this automatically - Bundling and Minification

Edit: A more detailed example using a regex

$guid    = [guid]::NewGuid()
$regex  = ".js|.css"
$replace = "?v=$guid"

Get-ChildItem -Path "C:\inetpub\wwwroot\Inet\MyApp\Views" | ForEach-Object {

    # store the filename for later and create a temporary file
    $fileName = $_
    $tempFileName = "$_.tmp" 
    new-item $tempFileName -type file -force | out-null

    get-content $_ | % {

        # try and find a match for the regex
        if ($_ -match $regex)
        {
            # if a match has been found append the guid to the matched search criteria
            $_ -replace $regex, "$($matches[0])$replace" | Add-Content $tempFileName 
        }
        else
        {
            # no match so just add the text to the temporary file
            $_ | Add-Content $tempFileName 
        }
    } 

    # copy the temporary file to the original file (force to overwrite)
    copy-item $tempFileName $fileName -force

    # remove the temp file
    remove-item $tempFileName 
}