my computer is running extremely slow, what can I do to increase the speed, and where do I go to delete the temp files, I might add I do not know my password to access the computer janitor

my computer is running extremely slow and freezes on a regular basis, I want to delete the temp files but I'm not certain where I need to go to complete this task. Someone else loaded Firefox onto my computer, I do not know my password so some of the functions needed, where do I go to access my passwords.