wei tong wrote:
I installed MC15 as administrator.
And I guess you had installed the preliminaries as well (MDAC 2.8 or higher, and the exact versions: .NET 3.5 SP1, MSXML 4.0 SP3). I'm not sure but I think in contrary to MC14 MC15 will complain at installation time if something is missing.
Unfortunately my "wisdom" is at an sudden end. 
I don't think that it would make sense to register other .dll's in the Mathcad15 directory. I just looked at my installation (WinXP SP3, too) and of the 120 dll's in the MC15 dir, only three are registered (automation.dll, glr.dll and mclauncher.dll) along with mathcad.exe and tabler.ocx.
BTW, I have used the free tool RegDLLView from www.nirsoft.net to get that information.
Getting in contact with PTC seems appropriate now.