There are many potential causes for an unusually slow computer. It could be caused by a lack of memory or storage space, outdated hardware, conflicting software, viruses or malware, or even a program running in the background that is sapping resources. To figure out why your computer is running slowly, you will need to inspect the system resource usage and look for any signs of problems. You may also want to use a tool such as a PC optimization program to identify any issues and provide helpful tips for improving performance.