A legacy computer system is one that is outdated in either terms of hardware or software. At the time of writing, any computer running a version of Windows below Windows 7 is usually considered to be a legacy system, because this is the latest version currently available. (This also applies to computers running earlier versions of any other operating system, such as Linux)
Unlike a lot of other items, which tend to last for a long time, computers seem to go out of date extremely quickly, which can cause problems if the equipment is not updated where necessary, especially within businesses.
This is because of a phenomenon known as Moore's Law, which is the prediction that every 18 months or so, computer capacity doubles.
For example, if the most advanced specification available is 1GB Ram with a 2Ghz processor one year, it would be double that value after 18 months.
I have found over the years that there are both advantages and disadvantages to using legacy systems.
One of the advantages is that some older programs will only run on older computer systems, such as some of the early games produced by various companies.
Another advantage of these systems is that, because they are so out of date, they are often cheap to obtain, and can there before be ideal for anyone who has never used a computer before, or who only needs it for basic functions, for example word processing.
Also, although legacy systems are older, they are also normally more stable. This is because update patches are usually released once a month from when the operating system is released to around 5 years later. (This only applies to Windows operating systems)
However, using one of these systems can often mean you get left behind rather quickly if you want to run any newer games and software or use any newer hardware, such as printers or scanners, which are not always backward-compatible with older systems.
This is primarily because as hardware capabilities increase, so do the demands of any software that is created, because developers always aim to take full advantage of new technology where they can, to enhance the user experience.
Overall, I would recommend that you consider what purpose you need a computer system for when choosing whether to use a new computer or an older system. In other words, for reliability, choose an older system and for higher performance levels, choose a newer system.