Oddly enough, this is related to how the executables are extracted from the Nant 0.91 archive. (This made no sense to me until I actually tried it, but it does actually work…)
Source : http://surfjungle.blogspot.com/2011/11/tip-running-nant-091-on-windows-7.html
I found that the problem was Windows 7 security related in that the downloaded NAnt 0.91 zip file needed additional security related configuration to be performed: before extracting, one must right click on the zip file, select Properties and under the General tab, click the button labelled Unblock, then click OK on the Properties window. Now, extract the file to your desired location, ensure it is on the system path, open a new command line and NAnt should run successfully.