I got my last three desktops from here: http://www.sohodiffusion.com/
One I assembled myself . The other two I asked them to do it for me: I chose the parts and they charged $50 to assemble the PC and install the OS and they did a clean job (they did a mistake on the last PC by not including the RAM I asked for though - it was actually more expensive than what I had ordered but not the right amount - I sent them a notice and picked up the new ram to install...and they told me to keep the other sticks as well). They also have some pre-assembled PCs at decent prices for those who don't want to chose parts one by one. Downside is that customer support if you live far away seems limited (ok as I live close by).
And beware if you order from a US website: you can end up with terrible custom brokerage fees in addition to the regular taxes (I hate UPS...)