Computers run very slow sometimes due to many reasons. If the total memory was occupied by many background programs running simultaneously along with the main application, the performance will be reduced. You can always make the main program to run and stop executing the remaining programs. This can be done by selecting all the other tasks in the task manager and then terminating them. When the system is rebooted, the speed of it can be restored. If the hard disk is full then it can make the system slow.
The system performance will be regulated by the C drive. As the cache memory of windows is in C drive, if it has unnecessary and unused files then the speed of the system gets reduced. When any program was loaded and then deleted repeatedly, then the hard disk can get fragmented which might cause the computer to become slow. This problem can be rectified by defragmentation of the hard drive. Low capacity of RAM can be a reason for the computer to be slow. In the case of low capacity of RAM, big applications cannot be executed efficiently and supported by the existing RAM.
If the computer has unnecessary network drives, has disorganized files and has many automatically running programs during the start up then the start up of the computer becomes slow. The performance of an old computer gets reduced as time progresses. The virus attacks might create problems in the computer and makes it to run slow. If the device drivers of windows were old enough and conflicting, then they might cause the system to be slow. Frequent alterations in system configuration and network configuration, wrong installation and un-installation procedures, and downloading of files can cause registry errors. The errors in registry can diminish the performance of the computer. Certain operating systems like windows XP have built-in tools to clean up the system and restore the performance.