Intro

 Purchasing a computer for your DAW to run on is not as difficult as you might think. The choices are plentiful and it is more of a matter of cutting through the marketing bla bla and get to the facts. If you have limited knowledge on computers reach out to one of your friends with computer skills that can sort out the facts for you.

Determine your requirements

 You will need to understand exactly what you would like to accomplish with your DAW. What DAW software you will use will play into most of the deciding factors. Most DAW software does exactly the same thing but might do slightly different from a procedural perspective. If you do not have a DAW of preference you should let your OS and hardware selections help determine your DAW selection.

Operating System Requirements:

   Operating Systems are the software that makes a computer do the basic tasks we expect computers to do. Most operating systems do the exact same thing. Some of selecting an operating system is a personal preference decision. Going with what you know is not always a bad idea. This article will focus on the facts behind each OS specifically around audio processing. There are trade offs with each OS so you have to know which of these trade offs are important to you.
   -Windows Vista: Is the latest flavor of Microsoft Windows. This OS is the most user friendly although at a trade off of performance and reliability. If you plan on using Vista you had better plan todouble hour hardware requirements to get it to perform. My advice steer clear of Vista for your DAW environment.
  -Windows XP:  Microsoft Windows prior to Vista. This OS is still available because of all of the performance and reliability issues with Vista. If you prefer Microsoft this is the better OS for a DAW environment although it is not as user friendly or flashy as Vista. Another benefit to Windows is that there are more applications available for Windows than any other OS. This is true with viruses too, so there is your trade off.
  -OS X Mac: Mac builds the best of both worlds for a DAW environment. Macs are user friendly and the OS processes audio much more efficiently than the Microsoft environment. Macs are also more reliable as the hardware environment is managed much more closely than that in a Windows environment and there are very few viruses that effect Mac platforms. The trade off is that Macs do come at a much higher price point so be prepared to spend some money. Macs do run on the same hardware architecture as Windows but only on certified branded hardware. Mac is based on the BSD flavor of Linux.
  -Linux: Linux is a UNIX based Operating System built for the PC architecture. Most Linux distributions are free unless support is needed. Linux outperforms Windows and is also more reliable than Windows but is not as user friendly. Linux based DAW applications are still in their infancy but do show signs of maturity. The advantages are that you can control much more of the Operating System making it more efficient but if you do not have a Computer Science degree you will struggle with accomplishing it.
    So this information does not settle the age old argument of which is better a Mac or PC but it should let you understand the true differences. This is the order I would rank the OS selection:
   1- Mac or Windows XP (Depends on budget and DAW preference)
   2-Windows Vista (As long as you don't care about performance)
   3-Linux (Give it a couple more years)

Hardware Requirements

    This section will be generic and not specific to any particular Operating System as they all run on the same hardware (For the most part). The focus will be primarily on the sizing aspect of the computing environment.  This will help you to identify what exactly is needed for you to get started. This information is intended to be very high level for ease of understanding.
 CPU: This is the central processor that makes the computer work. The CPU can be sized by 3  measurements, clock speed, Bit depth and quantity. Newer chip models also include "cores" as a measurement. A core is the same as number of CPU's (kinda). So a 2.0Ghz, 64 bit quad core processor is like having 4 64 bit processors clocked at 2Ghz. 64 bit is the amount of data the CPU processes at once. So if you plan on recording at 192k samples at 64 bit floating point a 64 bit CPU would be best. The number of CPU's and the core count is important, if you plan on using software based plugins the more CPU's you have the better.  Read your system requirements for your DAW software to make certain you meet the recommended specs. 
 RAM: Random Access Memory is the amount of data a computer can process without reading or writing to the disk (kind of). RAM is important so alway go beyond what the recommended minimum is for your DAW software. If the system uses all of the RAM it creates new RAM on the hard drive called SWAP, this hard drive based memory is much slower than RAM. If you start off with the minimum recommended RAM be certain you have available DIMM sockets to expand your RAM later. Vista users, remember to double what you think you need.  
 Hard Drive: This is the device that stores all of the data on your computer so when you turn it off it doesn't go away. When selecting a Hard Drive speed is more important than size. The more data you can read and write means the more tracks you can record or master or the higher quality data format you can use. Serial ATA is the most common disk interface today and will work just fine. Do not use a USB disk as they are slow and will compete with other devices for data throughput. USB disks are OK for backups but not as a primary medium to record your digital adio to.
 Interface Options:This is all of the buses and or connections to connect other devices to. Make certain you have the following: USB 2.0, Firewire , PS2- Mouse/Keyboard, 100M Enet or better, 2 or more open (166M) PCI slots. This is where you have to be concerned with future expandability. You will need USB and Firewire to connect control surfaces, audio capture devices, etc. PCI slots are important for audio capture devices, DSP processors (UAD) etc. Make certain you have room to grow.

Minimum Specs: 2.0Ghz or better 64 bit 2 core with 1G RAM or better.