If you were after a powerful PC that runs Windows 7, is a media center, and runs the latest games, I would recommend you build something like this:
Robertsmalls' 23W (at idle) computer - EcoRenovator
Note NiHaoMike's posts in that thread, talking about non-x86 PCs whose power consumption puts even the best x86 to shame. It would be pretty fruitful if you could migrate to such a platform.
Antique components aren't of any use to you. Old chips are built on larger processes, run at higher voltages, and lack the power management features of newer components.
Here's what I'd build:
*Motherboard with 760G. Look around on SilentPCReview.com and find one with good undervolting options. Underclocking and undervolting is the key to low power consumption. Integrated graphics saves you from having to power a discrete video card.
*Sempron CPU. Get the slowest 45nm single-core CPU you can find, then underclock it as low as you can stand, then undervolt it to the threshold of stability. Since you're talking about Windows 9x, I assume this means a 2.7GHz Sempron running at 800MHz.
*A small stick of RAM, undervolted and underclocked.
*An SSD if you're feeling spendy, or a single-platter 5400RPM hard drive if not
*A small LCD monitor with the backlight set to minimum. Build the monitor into a recessed shroud if that's what it takes to lower the brightness. My television uses 150W at full brightness, and 45W at minimum. My 19" LCD uses 30W at full, 10W at minimum.
*PicoPSU. There's nothing more efficient on the market, so there's only one choice in PSU.
That's everything! Don't allow yourself any unnecessary peripherals like floppy drives etc. The whole system, including monitor, should be around 30W.
Or just buy a cheap NetTop, throw it under the desk, and hook it up to a real monitor, keyboard, and mouse.