I used to build all my computers. Remember setting the jumpers to match the voltage of the CPU? Scary stuff if you got that one wrong. Jumperless motherboards sort of took the fun out of it. But, if you can hook up a surround-sound system, you can easily build a PC.
When my last computer was on its last legs, I broke down and got an Acer from Newegg. Fantastic machine. I've had it for nearly three years and, after upgrading the video card, I see no reason to replace it. Intel 2.4G quad core, 4G ram, 320G HD, DVD reader/burner with lightscribe, multi card reader. At the time, a fairly good machine (except the on-board graphics) that I think I got for less than $400. I've since added a second 500G hard drive.
I've always felt that PCs slow down over time because of software installations. For instance, Nero and some other burning software add a whole host of services that eat system memory with very little benefit. At one time, both Nero and Vista (
) were indexing my hard drives. Unnecessary. I got rid of Nero, turned off indexing, and use Imgburn to burn everything. I have two printers, both of which ran services in the background. I turned all of them off and reclaimed my memory (and I can still print).
As a favor, I helped someone with a PC. They had so many services running that they had no memory left. Older machine, running XP, but all the memory was the swap file. We shut everything off and it was like a new machine ... well, new as in as fast as when they bought it.
Sorry, got off topic. Building a PC is fun if you have the time. But, since the computer manufacturers can't prey on the ignorance of the users any longer, the price differential has tightened a bit.