Recently, I've had to help my brother with his computer because it runs slow. If someone at Best Buy is telling you that you need a new computer ask them why they haven't directed you to tools to make your computer run faster first. I recently had this experience at Best Buy. The guy kept trying to upsell me on a new computer when I looking at the specs on the products to see the latest optimizations. You really don't need a new computer unless you bought the bottom end model and it's over 6 - 8 years old. Advances in technology come in waves with long periods of minute optimization.
Below is my stack to get your computer running like new again.
1. IoBit Uninstaller
This is a free uninstaller that doesn't have a trial period for force uninstalling like Revo Uninstaller. When it gives you the option to do a deep scan after it uninstalls use the deep scan to remove registry entries. This program will probably install Advance System Care with it. You don't need that. Use IoBit to remove the program after you've done all the installing you need.
2. CCleaner
This is a conservative program that does a lot of crap cleaning with junk files etc. Two things: Run the regular cleaner and run the registry cleaner. If you haven't cleaned in a long time this will take hours. CCleaner does MFT cleaning, which takes a while. You'll definitely want the PRO version, which comes with a 14 day free trial.
3. Privazer
This is a privacy cleaner. Run it after CCleaner. It's a trace cleaner that does similar things to CCleaner and IoBit Uninstaller. However, it cleans up the registry entries left behind by both of them. It has a pretty simple interface that's hard to fuck up. For people concerned about their internet privacy this a great tool to use after you have preventative measures in that prevent your tracking. The data is wiped back to 000's in memory.
4. Auslogics Disk Defragmenter
Defragmenting is one of the last steps to cleaning your PC. It's necessary because an organized room is a room you can find things fast in. Aka. your computer definitely needs this if you've never done it before.
5. CCleaner
You're not cleaning again this time. This time you need to use CCleaner to clean up your startup items. This is critical to a faster computer. RAM is used up before you even get a chance to run anything. Most slow computers are slow from tons of startup items. If you're not sure about a process then look it up on the internet.
You should have a computer that runs like new for the next year. Pat yourself on the back. You basically did what someone else would have done if you had taken it to an electronics servicer to make it run faster.