The main causes of a slow computer are:

- Spyware
- Corrupt and Erroneous Registry Entries
- A Combination of too many Memory-Intensive Programs and Insufficient Memory (RAM)
- Fragmented and/or Cluttered Hard Drive

The main cause of a slow startup is having too many programs configured to run automatically when Windows boots. A system can only juggle a limited amount of tasks at once, depending on the amount of memory installed. An installed program takes up space on your hard drive, but it also uses memory when it is being run.

Any application that does not uninstall properly, or does not have an uninstaller, can leave entries in the Windows Registry. These can build up over time, and lead to decreased system performance and stability. Furthermore, in most cases, uninstalling a program on one user account will not remove the Registry entries in every other user’s account configurations.

The Windows Registry is organized much like the Windows file system that is viewable in Explorer. At the top of the hierarchy are the main branches of the Registry, and each of these categories is called a Hive. Hives are composed Keys which can contain other Keys, as well as Values.

The Windows Registry is a central hierarchical database used to store information and settings for users, applications and hardware devices. It contains information that applications and system processes constantly retrieve and store during operation, such as a profile of user’s custom configurations, the types of documents that applications can open and create, as well as the hardware and network ports that exist and are being used.