|If you had alittle more power in that PC (1.5Mhz) I would recommend getting the OEM Version of MS Home Server. It is basically Windows 2003 with some added functionality with all the really techie stuff stripped out (e.g. domains).|
I do not run Vista at home, so I can't comment on that. I ran a File Server for a couple of years using XP on an old 550Mhz system and had no problems. I later upgraded it to Win2003 and have been running it fine ever since (Well, except for the motherboard dying a few weeks back, but a $20 replacement was had on Ebay).
However, Win2003 is not cheap, so I would suggest using XP - but apparently there are some problems with XP playing nice with Vista according to jefro's response. But, that would probably still be my choice.
There's always Linux, but there goes the "easy" part of the equation.