How to work with registry default values

To read the default value of a key :

(get-itemproperty -literalpath HKLM:\SOFTWARE\Classes\.txt).'(default)'

To create a default value :

New-ItemProperty -LiteralPath HKLM:\SOFTWARE\Classes\.txt -name '(Default)' -Value "txtfile"

To set an existing default value :

Set-ItemProperty -LiteralPath HKLM:\SOFTWARE\Classes\.txt -name '(Default)' -Value "txtfile"