Log-on time

I have 3 computers on a network (2xVista,1xXP) with a large number of shared
folders and subfolders. I noticed when I boot (especially) the XP PC while
the two other PC's are off, that the time it takes to finally getting logged
on (icon in notification area) is significantly shorter than when the
Vista's are operating. Is this normal?..Something wrong with the XP network
driver? Thanks for any advise.