For non-notebooks, I still assemble from scratch. It's the only way to avoid having to pay for a copy of an operating system that I really REALLY don't want.

And since I purchase wholesale, the prices are quite reasonable, and I get to customize with the exact specific brands/models of components desired. Ditto for ongoing upgrades and the like.

This is particularly handy for the MythTV boxes -- an off the shelf Dell or something would simply lack the required expansion room for drives, tuners, VFD front panel display, etc..

For notebooks, I buy new/used according to what's available at the time, and obviously don't assemble those myself. The useless copy of Windows that I'm forced to pay for ends up sitting in the shrink wrap until the unit is resold years later.

Cheers