I wouldn't say it's as easy as LEGO.
There are a few things you need to be aware of:
1) Don't push *anything* -- even the slightest contortion of the CPU or mother board, etc and and you'll render the hardware useless and it becomes a paper weight at best.
2) When building from scratch, try and have the guys in the computer shop install the motherboard and CPU. Most parts plugin easily but the motherboard usually requires some interesting cabling. There are wires for hard drive lights, for speakers, etc. Those are sometimes tricky to find on the moterhboard (especially if you don't have the docs) and in my experience polarity does make a difference -- get it wrong and if your lucky nothing happens, if your not lucky, you fry a component.
The biggest mistake I ever made assembling a computer, was installing the motherboard *without* the metal/plastic standoffs -- easy to make if you've never assembled a PC before and think it should be easy peasy. Once you apply power, you fry the board. I salavaged my CPU and video card, but the memory sticks were shot.
3) Static. Most people assemble computers without any regard to static. I've assembled tonnes of computers and fixed even more and never had an issue. Then one day, on a cold, dry winter afternoon I attempted to fix a friends computer, while standing on carpet and snap. I felt a small shock and when I booted again, the computer wouldn't boot. It had booted only seconds earilier just fine. I eventually concluded that my little static shock destroyed the memory chips. Bad memory on this particular ASUS board caused failure to boot. I removed the memory and voila -- good as new but never again did I work on carpet.
The moral of the story, touch something metal before you go tampering with computer parts, at least you will discharge somewhat and not pass that onto your new computer parts.
4) Lastly, there is the issue of understanding the various configurations of computer hardware. When you install a CDROM and hard drives, etc, you better make sure you have the jumper pins properly configured, because when you boot and your HDD doesn't show up you'll be stuck wondering why.
Do you know when to set the jumper pins to primary master, primary slave, secondary master, secondary slave, cable select?
It's not difficult, but it won't be mentioned in the docs you get with the PC -- it's assumed you already know.
It's little things like this that can make assembling your own PC a real hassle and PITA.
I would suggest, having the techs install the motherboard, CPU, RAM and video card.
In fact, it should be store policy to install at least the CPU and RAM on the moterhboard and they should show you that it works and boots fine. Otherwise what is common, is people go home, install the CPU, puch to hard and crack the unit and come back to store complaining how the computer doesn't work. At this point it's to late for the store to return the part and they have no choice but to replace a part faultyed by human error -- or tell you to go away.
Installing hard drives, CDROM, video cards, etc is pretty easy. It's plug and play technology, but you might need to understand the configuration of jumper pins, unless your lucky and the manufacterer releases the disk already configured and it matches your setup.
Otherwise, just goto your local computer store, ask the sales rep to sit down with you and go over what you need -- it's a PITA getting home only to realize you missed a component, like a mouse or something.
Certain parts are only compatible with certain moterhboards, etc. If you buy parts willy nilly and assemble yourself, you might find yourself with incompatible parts and be stuck with them.
Go over the list of requiresments, but the parts and just have the tech install everything for you, it's waaaaay less hassle free and you leave with a machine that works. It costs maybe 100 bucks and take only 15 minutes.
It's still better than brand name computers because you don't get any garbage parts: I recommend:
ASUS motherboard
AMD CPU - forget Intel
Kingston RAM
nVidia or ATI video card
Seagate or Western Digital hard drive
LG monitor
Don't bother with a wireless mouse or keyboard -- as a developer I find myself changing batteries every day and when my keyboard battery dies -- my computer freezes for several minutes. Likely a driver bug and easily patched but I'm lazy and it's still annoying.
Oh I wouldn't bother with a dual or quad core processor either, if all your doing is development or general computing. Gaming maybe or a server, for sure, otherwise save your money and buy a single core. Get a quality heat sink and ask for that liquid metallic goo that goes between the CPU and heat sink -- it'll make a big difference.
Buy Windows XP pro and stay away from Vista -- it's such garbage its rediculous.