An existing computer system is an obsolete system in terms of hardware or software. At the time of writing, any computer running a version of Windows running Windows 7 is generally considered a legacy system because it is the latest version currently available. (This also applies to computers running earlier versions of any other operating system, such as Linux)
Unlike many other items, which tend to last a long time, computers seem very quickly obsolete, which can cause problems if the equipment is not updated if necessary, especially in companies.
This is because of a phenomenon known as Moore's Law, which is the prediction that every 18 months or so, double computer capacity.
For example, if the most advanced specification available is 1GB Ram with a 2Ghz processor a year, that would be double that value after 18 months.
I have found over the years that there are both advantages and disadvantages to using existing systems.
One of the advantages is that some older programs only work on older computer systems, such as some of the first games produced by various companies.
Another advantage of these systems is that, because they are too old, they are often cheap and can be ideal for those who have never used computers before or who have not used them before. Only need them for basic functions. eg the word processor.
In addition, although older systems are older, they are also normally more stable. This is due to the fact that update patches are usually published once a month from the time the operating system is released about 5 years later. (This applies only to Windows operating systems)
However, using one of these systems can often leave you pretty quickly out of the way if you want to use newer software and games, or use newer hardware such as printers or scanners. which are not always backward compatible with older systems.
This is mainly due to the fact that hardware capabilities are increasing, just like the requirements of any software created because developers are always looking to take full advantage of new technology where they can, to enhance the experience user.
Overall, I would recommend that you consider for what purpose you need a computer system to choose whether you want to use a new computer or an old system. In other words, for more reliability, choose an older system and, for higher performance, choose a newer system.