The big question you have to answer first - do you want to build one.
To build or not to build, that is the question.
The advantages of building
1. Lower initial cost on most systems, although might be minimal on basic systems costing less than about $450
2. Personal satisfaction
3. Better quality components
4. Acquisition of knowledge about computers and skills
5. Confidence in working on computer - and in doing upgrades
6. The big cost savings really comes after several years when you can upgrade instead of buying a new one. You can generally continue using the case, power supply, optical drive and operating system (depending on obsolesce on the latter) and maybe the graphics card (which you might already have updated anyway), and get just a new motherboard, CPU and memory at about half the cost of a new machine.
7. No vendor installed bloatware slowing down your system and taking up hard drive space for the life of the computer.
8. Having a complete BIOS that allows making changes and supports overclocking rather than one limited by manufacturer.
9. The ability to size components correctly so you don’t later find out that when you want to upgrade a graphics card that you also have to upgrade and replace a power supply.
10. Membership in the eclectic group of BYOers - a very intelligent, affable, handsome, honest, trustworthy, loyal, kind, and modest group.
11.Bragging rights - be they as they may - of BYO - and all the chicks it brings - or vice versa if you are vice versa - or even just vice.
Of course there are disadvantages:
1. Time is the big one - you have to invest some time in configuration, purchasing components, assembling the PC, loading the operating system, testing it, and sometime trouble shooting problems. The actual assembly only takes about 1-2 hours for an experienced hand, for a newbie taking their time approximately 4-6 hours, assuming no problems, which do occasionally occur. But if you consider it a hobby and learning experience then this should not be a big issue.
2. Support and - if something goes wrong with the PC you don't have a convenient number to call, you have to fix it yourself, with help from forums like this.
3. Warranty - you have the individual component vendor warranties which are sometimes shorter than what is provided by a vendor selling complete systems.
4. No vendor to cuss at when things go wrong. If you enjoy being able to rage at someone for months the BYO is not for you – it is no fun raging at yourself.
Here’s how simple it really is:
Youtube video showing how easy it is – with good general instructions
http://www.youtube.com/watch?v=D_E3ULURHkE&feature=related
Step by Step Instructions with Pictures
http://www.computerforum.com/104641-how-build-computer-step-step-photos.html
http://www.tomshardware.com/forum/274745-31-step-step-guide-building
And two more options with detailed instructions:
http://en.wikibooks.org/wiki/How_To_Assemble_A_Desktop_PC/Assembly
http://www.kitchentablecomputers.com/index.php
For more detailed instructions on installing a video card:
http://www.upgradevideocards.com/install.html
Off course, most computer components also come with instructions for their installation, and the motherboard usually has a guide for plugging everything into it.
Now go back and re-read the advantages and build that thing. Or not.