When you buy a computer from new if it does not break down within the first few days of working it should last a lengthy time.Older models just need one component to fail. What can go wrong with Computers there are a number of reasons. After building Computers over a number of years, they have become so cheap to buy as a package that unless you want a customt  built one its not worth the time to build one unless you enjoy doing it. If you find  the Computer stops working and starting after a short time or slows right down, this could be the software, of the files in the holding area, this Delete all temporary files and cookies, or at worse Reformat Dics and reload software. Or maybe the cooling fan for the  Proceesor is failling, replace the cooling fan. The other common thing I have found on all computers over time (2-3 years in use) is the Time clock on the computer slows down, this is caused by the motherboard battery, on my last 3 computers I have used a Lithium 2032. Very easy to do but take all your data off the computer before you do this as the Bios will reset its self to default and Windows XP software may detect new hardware and not work correctley after replacing the battery, if unable to recover your opperating system then it will be a format the Hard drive and reload the opperating software.If you are not sure of what you are doing then it is always best to get expert help.  Thank you for reading this Hope it may help