You need a registry editor to do that. Quite a few on freewareppc.com. Try this one if you don't have one:
http://www.freewareppc.com/utilities...ryeditor.shtml
And looking at that key I believe it should be
HKLM\Comm\Cellular\SMS\Protocol\Text\DLL
and changing HTC_SMS_Providers.dll to SMS_Providers.dll
namopereht