This is the code under XP, under Win78 is useless,
Who give a support Win78 code, thank you, as long as the code, not theory, thank you)

 bool __stdcall WriteStringToReg (hkeys Root, AnsiString Key, AnsiString KeyName, AnsiString KeyValue) 
{
Bool OK=false; 
TRegistry * Registry=new TRegistry (); 
Registry - & gt; RootKey=Root; 
If (Registry - & gt; OpenKey (Key, true)) {
Try {
Registry - & gt; WriteString (KeyName, KeyValue); 
OK=true; 
} 
The catch (... ) {
} 
} 
Delete the Registry; 
Return OK; 
} 
//-- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
Bool __stdcall RegFileType (AnsiString Ext,//extension, such as: "dyc" 
AnsiString ExtTypeName,//file types, such as: "dyc file 
"AnsiString AppFileName,//application name, such as "c: \ \ dyc exe" 
AnsiString FileDescription,//file type description, such as: "dyc documents 
"Int IconIndex)//icon indexes, such as 0 
{
Bool RegOK; 
RegOK=WriteStringToReg (HKEY_CLASSES_ROOT \ \ "" + Ext," ", ExtTypeName); 
If (AppFileName. Pos (" ") & gt; 0 {
AppFileName="" "+ AppFileName +" \ ""; 
} 
If (RegOK) {
RegOK=WriteStringToReg (HKEY_CLASSES_ROOT \ \ "" + ExtTypeName," ", FileDescription); 
} 
If (RegOK) {
RegOK=WriteStringToReg (HKEY_CLASSES_ROOT \ \ "" + ExtTypeName +" \ \ DefaultIcon ", "", AppFileName +", "+ IntToStr (IconIndex)); 
} 
If (RegOK) {
RegOK=WriteStringToReg (HKEY_CLASSES_ROOT \ \ "" + ExtTypeName + \ \ Shell \ " Open \ \ command ", "", AppFileName +" \ "% 1 " "); 
} 
Return RegOK; 
}