[semiono]

Код:

function x(): Boolean;
begin
  if RegValueExists(HKCU, 'Software\a', 'b') then
  begin
  RegDeleteValue(HKCU, 'Software\a', 'b')
  end
  else
  begin
  Result := false
  end
end;

function y(): Boolean;
begin
  if RegValueExists(HKCU, 'Software\a', 'b') then
  begin
  RegWriteStringValue(HKCU, 'Software\a', 'b', '0')
  end
  else
  begin
  Result := true
  end
end;

[Registry]
Root: HKCU; Subkey: "Software\a"; ValueType: string; ValueName: "b"; Flags: deletevalue noerror; Permissions: everyone-full; Check: not y

Root: HKCU; Subkey: "Software\a"; ValueType: string; ValueName: "b"; ValueData: "0"; Flags: noerror; Permissions: everyone-full; Check: not x

[Files]
Source: "1.TXT"; DestDir: "{sd}"; Check: x
Source: "2.TXT"; DestDir: "{sd}"; Check: y