Not exactly sure what you are trying to do. If you want an FFT of a window sliding over time data the following webapage contains an example:
http://zone.ni.com/devzone/devzoneweb.nsf/opendoc?openagent&1BD6D3BDB8BD0F4F8625696D00463B63&cat=F64E0FD3F923510C862567F5007AA89C
NI also provides an advanced signal processing toolkit:
http://www.ni.com/analysis/joint_time.htm