There are several causes that can affect the PC’s performance such as registry database packed with junk keys, outdated system drivers, junk files in system drive and more.

Try the given fixes to fix slow computer uninstall the crapware and go to task manager and check the process that are consuming high CPU usage and kill them.

Despite this also scan for viruses and malware at the regular interval of time with the good antivirus program.

source: http://www.pcerror-fix.com/19-commonly-reported-windows-problems-fixes