I've never used moo.dll, but whichever portion returns the os (let's assume it's $dll(moo.dll,osinfo,_) for this example), just
var %osinfo = Windows XP Professional, Service Pack 1 (5.1 - 2600)
var %os = $replace($dll(moo.dll,osinfo,_),%osinfo,WinXP)
Or just don't call moo.dll for that, just use the string literal instead. Where you would call $dll(moo.dll,osinfo,_), just put, literally, WinXP