Yep - I have the same 'problem'. I have a Lakeport 945P chipset in my Dell: List of Intel chipsets - Wikipedia, the free encyclopedia
And as the Note on the Wikipedia page states:
'Lakeport chipsets lacks support for remapping memory; the size of addressable memory space may be less than 4 GB, regardless whether the processor operates in 64-bit mode.'
For me, Windows actually reports 4GB installed physical memory, but then goes on to state 'Total Physical Memory = 3.581GB'.
Not that it affects me. The 945 chipset is the same reason I can't run anything more than a Pentium D. I have maxed out this PC, but the performance is more than enough to give a very good experience with Win 7, particularly now I have fitted my SSD