Normally any computer is slow as first step delete the following older content.
1. Temp folder (unnecessary content of files)
2. %temp% folder (unnecessary content of files)
3. Prefetch folder. (unnecessary content of files and logs files)
Step-by-step explanation:
Once above folder content is deleted. Restart the computer. After restart computers, update operating update up to latest patches.
Update latest patches for anti-virus .scanning the computer or desktop to find any virus effected.
Download third part malware and spyware software and scan the computer or desktops to find any desktop or computer is affected for spyware or malware.
Remove unnecessary software from each computer or desktops.