There are many computer repairs that the average person can perform without much problem. Things like installing CD-ROM drives, sound cards and other expansion cards don't take much computer knowledge. How can you know though when you are over your head and need professional computer help?
Some computer repairs are difficult for the average person to do without causing damage to their computer or a peripheral. There are some repairs I'd classify as beginner level, some as intermediate and others as advanced. I've already mentioned some repairs I would call beginner level. Some intermediate repairs include replacing your hard drive and installing an operating system. There was a time when something like this was advanced but with modern Microsoft operating systems like Windows XP and Windows Vista, operating system installations are not as advanced as they used to be.
So what repairs should a beginner not attempt? I would suggest some things like replacing your motherboard. The need to replace your motherboard can come either when you are ready to upgrade your motherboard to something newer or when your computer case power supply kicks the bucket.
Another repair I would caution against is based in software. That's when you need to clean out a nasty virus or spyware program. It is very easy to cause you much grief if you don't do it right. You could lose access to the data on your hard drive if you mess up your operating system. Now if you don't care about the data on your hard drive, simply re-installing your operating system while making sure to format your hard drive can usually solve a software problem. Kind of like killing a fly with a large shovel. It works but it is overkill. A professional computer repair technician can help you keep your data while ridding you of a virus infection.