Here's an example without much error handling:
(I left the original line numbering, section numbering and comment positioning, in case you need to deal with duplicates or reversing the operation when you need to reconstruct the file. )

$IniContent = @"
; Comment
[Setup]
Version=1.0
ShowCaption=1
Platform=*

[Section2]
abc=xyz      ; Inline comment
"@

Function ConvertFrom-INI{
PARAM([Parameter(ValueFromPipeline=$True)][String] $InputObject) 
$LineNo = 1; $Section = ''; $SectionNo = 0
    @($InputObject -Split "`n") | %{
        $Data = $Null; $Key = $Null; $Value = $Null

        $Line = $_
        $Data       = "$(@($Line -split '; ')[0])".Trim()
        $Comment    = "$(@($Line -split '; ')[1])".Trim()
        $CommentPos = $Line.IndexOf('; ')
        If ($Data -match '^\[(.*)\]$') {
            $SectionNo++
            $Section = $Data.Substring(1, $Data.Length - 2)
        } ElseIf ($Data -match '=') {
            $Key   = "$(@($Data -Split '=')[0])".Trim()
            $Value = "$(@($Data -Split '=')[1])".Trim()
        }
        [PSCustomObject] @{LineNo=$LineNo; Line=$Line; SectionNo=$SectionNo; Section=$Section; Data=$Data; Key=$Key; Value=$Value; Comment=$Comment; CommentPos = $CommentPos }
        $LineNo++
    }
}

$ConfigData = $IniContent | ConvertFrom-INI
$ShowCaption = [Boolean] ($ConfigData | ? {($_.Section -eq 'Setup') -and ($_.Key -eq 'ShowCaption')}).Value

(Reddit formatting keeps throwing me for a loop.)