Detection Method - powershell - registry

js2010's answer works well, even in older versions of PowerShell, but it requires duplication of the value name.

In PowerShell v5 or higher you can use the Get-ItemPropertyValue cmdlet instead, which avoids the duplication:

$key = 'HKLM:\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall\xxx'
$valueName = 'DisplayVersion' # in PS terminology: *property* name 
$valueData = '7.7.1' # in PS terminology: *property value*

if ((Get-ItemPropertyValue $key $valueName) -eq $valueData) {
  "installed"
} else {
  "NOT installed"
}

If, instead, you're looking for specific property value (registry-value data) in a given property (registry value) among all the subkeys of a given registry key path:

$key = 'HKLM:\SOFTWARE\WOW6432Node\Microsoft\Windows\CurrentVersion\Uninstall'
$valueName = 'DisplayVersion'
$valueData = '7.7.1'

if (Get-ChildItem $key | Where { $_.GetValue($valueName) -eq $valueData }) {
  "installed"
} else {
  "NOT installed"
}