Installed Physical Memory/Total Physical Memory

I just built my first pc running vista home premium x64 sp1. I have 8
gigs of ram installed.Bios sees all 8 gigs.When i display system summary
in vistas system imfromation it states i have 8 gigs installed
memory,3.75 gigs total physical memory,and 6.57 available physical
memory.Can someone please explain to me what this means.Should i have
more than 3.75 gigs of total physical memory? Do i need to
reinstall?Thank You.