The majority of applications that require the intensive use of the graphic card or of the sound card need DirectX to work. For older systems, it's advisable to download DirectX 8.
DirectX 8 is especially necessary for almost all games developed for Windows systems, due to their high-quality graphics and sound.
Version 8 of DirectX is capable of working on old operating systems: Windows 98, Windows Me, Windows NT and 2000. Making it ideal to work with low power equipment, like netbooks and nettops, even though we'll find it necessary to have a newer version to be able to launch more modern multimedia applications and games.
If you think that your computer isn't powerful enough to be able to play on it, try out DirectX 8, and you might be surprised.