The main problem seems to be simply running the 32bit Windows and being held back by the limitations on that. If all dimms are matched and in good working order without any defects the post tests and the system properties not the taskmanager should indicate the actual total of physical memory installed.

The SP1 for Vista being the previous version then allowed the full 4gb or more to be listed in the system properties while the ram available on the 32bit Windows there was still limited with anything over 3.5gb simply being mapped out to hardwares and unusable in Windows.