RE: Dell Inspiron 15 3558 SLOW
I'd also suggest you download free version of Malwarebytes/ Anti-Malware, and then run it. Malwarebytes is the best for getting rid of nasty malwares which will slow down the system.
Download free version of CCleaner, and then run it. CCleaner should help speed up your computer. I'd suggest you first click on cleaner, registry, and then tools. In startups, disable any programs that might be slowing down your system.