The main reason for buying a pre-built computer is the price. Today especially, you can purchase a base model pre-built computer at most retail stores for around $400, complete with everything that the average user would need. Quite often, they include a monitor, speakers, keyboard, and in some instances, a printer. Also, as a way of enticing consumers to buy, most pre-built computer manufacturers, also known as Original Equipment Manufacturers (OEM), such as Dell, HP, Compaq, Gateway, Acer, Asus, and e-Machine, will often bundle software so that you can begin using your computer right out of the box. Finally, you can usually contact a vendor for warranty repairs, if the computer misbehaves during the warranty period. Unfortunately, that's where the advantages end.
There are a couple major disadvantages of buying a pre-built computer. The first is that more and more computer manufacturers are using proprietary parts. In many cases, this shouldn't be an issue. However, I'm sure that we have all heard of something that has had a major problem after the warranty expires, and computers are no exception. It's when you attempt to replace or upgrade a part that you will find yourself at the mercy of the OEM. I have known OEMs that have charged customers as much as 5 times the cost for a replacement part. In other words, a part that may cost $20 for a custom-built computer could easily cost over $100 from an OEM. You may also find that a failed proprietary part is no longer available. I have found this to be true with failed power supplies and a 3rd-party replacement may not always be available. Furthermore, support is generally provided for OEMs via an off-shore help desk, usually in India. Also, some 3rd-party support vendors will decline repairs depending on the proprietary part that is involved. As a result, you may find it difficult to find a local repair shop and you may have to ship the computer back to the OEM for repairs (and expect longer turn-around times in most cases).
I recently took in an out-of-warranty desktop and laptop that both needed new motherboards due to overheating. The cost of repair would have been more than the replacement cost of these systems.
The second disadvantage is that you cannot be sure about the integrity of the parts used. Some OEMs have been known to integrate substandard parts into their computers because of the substantial discounts that they receive for these parts. If the computer passes the OEMs benchmark and quality test, the machine will be shipped to the end user with a refurbished part installed. In my 29+ years of experience with computers I have seen OEMs that provide users with a very nice and well-built computer case, but will include a cheap motherboard along with a proprietary power supply. One well-known OEM often develops systems that require proprietary memory and, in some cases, proprietary hard drives. These cheap parts will increase the chances of system failure. My advice: if you plan to buy a pre-built computer, make sure that the OEM has a solid reputation and avoid the low-cost configurations that are often pushed on customers at most retail locations.
The main advantage of buying a custom-built computer is that the system can be built exactly to your needs or specifications, unlike OEMs where you have to adapt to what they give you. In custom built computers you can specify everything from the brand name of the motherboard to the type and size of the case. Every part in it is standard so that repairing and upgrading it is faster, easier, and less expensive.
Custom-built computers give you the option of having a system built that fits your budget and, if you elect an inexpensive motherboard, it's your decision. Support can generally be obtained from most local 3rd-party sources or directly from the parts manufacturer. The upgrade path will generally be easier and less expensive.
The disadvantages of buying a custom-built computer is that generally there is no additional software bundled with the computer beyond the operating system. Buying additional software can significantly increase the price of the computer, of course.
If your work is "mission-critical", you may actually save money by buying a custom-built computer rather than buying a high-end OEM system. If you are using graphic-intensive applications or online gaming software, you may be better-served by buying a custom-built computer.
Whether you buy or build your computer, support should ultimately be available from a number of sources. However, support for proprietary systems may be hard to find for out-of-warranty pre-built/OEM systems. The standard warranty for a new pre-built/OEM computer ranges from 90 days to 3 years whereas warranties apply to the individual parts for custom-built computers.