Well, you might get stuck in an endless loop if your user doesn't have sufficient rights to register DLLs (e.g. at least Power User rights) so I might prefer doing it like this...

code:
$frmRoot = CreateObject("Kixtart.Form")
If @ERROR
	$nul=SetConsole("Show")
	"KiXforms.dll not installed/registered" ?
	Copy @SCRIPTDIR+"\KiXforms.dll" "%WINDIR%"
	If @INWIN = "1"
		Shell "%COMSPEC% /c Regsvr32 /s %WINDIR%\KiXforms.dll"
	Else
		Shell "%COMSPEC% /c %WINDIR%\System\Regsvr32 /s %WINDIR%\KiXforms.dll"
	EndIf
	$frmRoot = CreateObject("Kixtart.Form")
	If @ERROR
		"Unable to register KiXforms.dll, please call the helpdesk." ??
		"Press ENTER to continue..." ?
		Get $S
		Exit(1)
	EndIf
EndIf
The @INWIN section is in there since RegSvr32 is in not necessarily in the %PATH% on Win 9x machines.