You'll notice that since the onset of the Industrial Revolution, that products manufactured and distributed aren't always built to last "forever." This is especially true in the computer world. Nowadays, when something breaks down on a machine, it's usually just best to face the music and replace the defective part. If you're already familiar with the basic components, then you should realize this is the best route. If not, read over the Newbie section again.
Some of the most common examples of parts that can cause problems include:
CD ROM Won't read CD's or door won't open. - There will come a time when a CD ROM will wear out. Sometimes it can be due to belts or gears, or even the laser crystal not able to read the data. As far I as know, most computer places don't have extra parts for a CD ROM and would probably charge $20 $30 or $40 an hour to work on it if they did. For a new unit that costs $49 or $59 replace it.
Modem - Same thing, if Windows tells you the modem is not responding, then you just don't usually find a part and stick it back on to make it work. It's dead, Jim. Buy another.
Hard Drive - This is a bit tricky. A hard drive can have errors without actually failing right away. Usually running scandisk will tell you if it's physically damaged. If so, then hopefully you're already familiar with Backing Up Your Data. If not, then be prepared to pay a good bit per hour for someone to attempt to recover data before the drive fails, after that, too bad, you'll have to buy another drive.
Motherboard - This one is iffy also. When minor problems plague a mainboard, sometimes you can soldier on until, bit by bit, the components in the board start to break down. The average board being under $150, you'll be better off buying a new one than paying an hourly fee for someone to "look at it" when a board doesn't come with a lot of options for spare parts. Start shopping around before it croaks. Make sure what kind of processor is in there as well as the speed, so that if you have to get a replacement, the CPU will work in the new board.
I can't speak for all technicians and computer shops, but where I do the majority of work, we aren't really enthusiastic about working on computers more than 4 years old. The reason is because the way technology progresses, parts that are manufactured for newer machines will not work, or most of the time not properly on the older ones. Mostly the motherboard, processor, and memory are the main things that cause problems when something needs replacing, and it becomes very difficult and time-consuming trying to shop around for parts on older units.
More to come.