Because it's fast, 100% stable and hasn't let me down since I first installed it.
Besides, I can't stand the new versions of Windows Mail (can't stand alternatives either).
with a bit of option tweaking, windows live mail looks just like outlook express
I know because i hated the new one too !
p.s. rest of the system looks good, go for the 8gig ram
or... mb/cpu/ram/psu/gfx upgrade for about 300 to 350