Don't forget the peripherals when building a computer from scratch.
At its simplest, a desktop computer consists of a motherboard, central processing unit, random access memory and hard drive, but that isn't really what most people think of when they think of a desktop computer. Most premade computers have a motherboard with additional integrated peripheral components like sound, video and Ethernet cards. When you buy a motherboard, it won't have all of those integrated peripherals. Purchase and install separate cards for these features. For the installation of software, a readable drive is also useful, a DVD, CD or floppy.
Motherboard and Processor
A motherboard has different slots depending on the type of peripherals its meant to work with, the most critical being the socket for the central processing unit. LGA sockets are Intel. AM3 sockets are AMD. You can't run an AMD CPU on a motherboard with an LGA socket. The most basic choice is an AMD or Intel CPU. Motherboards also have different card input slots, either PCI, PCIe or AGP for video, sound and Ethernet cards.
Drives
While technically not a necessary component, most people need a readable drive of some type to import and export data. Motherboards with floppy drive inputs are becoming rare. A computer needs at least one hard disk drive if you plan to install software. Regular hard drives have four main features: size, rpm, cache and connection type. The only feature that determines compatibility with your motherboard is the connection type: SATA, PATA or IDE. IDE drives are being phased out. SATA drives are more common. There are different generations of SATA connections, but most are backwards compatible, meaning that you can run a first generation SATA drive on a motherboard with second or third generation SATA connections.
Memory
RAM is by far the trickiest part of building your own computer. The manual that comes with your motherboard will have a list of supported RAM types (such as DDR2 and DDR3) and manufacturers. Some RAM not on that list will still work, some on the list won't. RAM sticks usually run in pairs. If you want to run eight gigabytes of RAM, you should buy two four-gigabyte sticks, or four two-gigabyte sticks, but mix and match different sticks together, such as a two with a four. The more RAM you have, the faster the system runs. Check the stats on your operating system, to see how much RAM the operating system will recognize before buying. Windows XP 32-bit Home Edition will only support 3.25 gigabytes of RAM, so buying more is pointless. Motherboards also have a cap on how much RAM they can support.
Case, Fans and Power
In theory, a computer case can be anything: a plastic container, a cardboard box, a fish tank. Most people just buy a computer case. Cases come in three different sizes: mini-towers, mid-towers and full towers. Mid-cases are standard. The other variable in a case is the motherboard form factor(s) it supports, ATX, microATX or AT. Cases come with stock fans, usually a front intake and a rear exhaust fan. These fans are standard sizes -- 80 mm, 120 mm, 140 mm -- replace them, if desired. Most CPUs come with stock coolers such as a heat sink plus a fan. If the CPU does not come with a cooling system, purchase one. If the case you've purchased doesn't come with a power supply, purchase one separately. To determine how much power you need, add up the wattage for all the installed components and buy a power supply unit that has approximately 100 watts more.
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