Hi Gunji,
Refractions Research, the company that builds PostGIS has not released a 64 bit version on windows for PostGIS 1.5.x. As soon as they do we will test and certify that version, in the mean time PostGIS 1.5.1 is available on Linux, it is also supported on windows 32bit but ArcSDE 10.1 is not supported on 32bit platforms anymore.
Sincerely,
Kasia