The following list contains the API functions easily seen from checking wtsapi32 in a tool such as Dependency Walker. They are all present in Windows XP Professional and this appears to be a subset of those exposed by .NET server.
WTSCloseServer WTSDisconnectSession WTSEnumerateProcessesA WTSEnumerateProcessesW WTSEnumerateServersA WTSEnumerateServersW WTSEnumerateSessionsA WTSEnumerateSessionsW WTSFreeMemory WTSLogoffSession WTSOpenServerA WTSOpenServerW WTSQuerySessionInformationA WTSQuerySessionInformationW WTSQueryUserConfigA WTSQueryUserConfigW WTSQueryUserToken WTSRegisterSessionNotification WTSSendMessageA WTSSendMessageW WTSSetSessionInformationA WTSSetSessionInformationW WTSSetUserConfigA WTSSetUserConfigW WTSShutdownSystem WTSTerminateProcess WTSUnRegisterSessionNotification WTSVirtualChannelClose WTSVirtualChannelOpen WTSVirtualChannelPurgeInput WTSVirtualChannelPurgeOutput WTSVirtualChannelQuery WTSVirtualChannelRead WTSVirtualChannelWrite WTSWaitSystemEvent