ccleaner & defraggler
you sure you dont have a virus
other reasons its slow too many startup processes & programs that run in the back ground,a program called quicklinks2 (be sure once installed you click on system tray icon & untick auto start up,no point adding more stuff that autostarts )can help determine what processes in task manager are unnecessary,you can then use msconfig to disable those processes from starting or kill them from the task manager

depends on the machine to, XP on a laptop, must be an old machine unless you put XP on a new model,

ccleaner.

Avast and MalwareBytes for virus protection.