The search in WIndows 7 does not take more than 50MB - all of which is in its own address space and does not influence the amount of memory available to Poser. The same is true for most (if not all) of the other windows components.
Windows 7 64, Vista 64 and XP64 all have 4GB address space available for Poser. During the render phase some of it is used as shared memory leaving about 3.5GB for loading scenes.
Before you reach that you probably run out of other resources in Poser - independent of the OS.

On the 32bit versions you have 1.5GB available or 2.5GB if you enable the /3GB switch