WTSOpenServer() WinXp SP2

[peko]

development environment

IDE:Microsoft VC++ 7.0
OS: Windows XP professional

I have developed an application which controls a remote desktop
by using the windows terminal service API

But during an open terminal session service,
I get the following error:

・WTSOpenServer() return NULL
・GetLastError() is 0x00000005(ERROR_ACCESS_DENIED)

[local security set up]
[security set up ]　
[local policy ]　
[assignment user right ] 　
[logon terminat service permit]

As the result of much trial and error I have discoverd that
I get this error when Windows XP Professional SP2 is installed.
When Windows XP Professional SP2 was uninstalled, processing
executes normally.
Are there any settings other than the above which I should be concerned
about?

Any advice is appreciated!