Keep in mind that all achievements unlock on close. They will show up as unlocked on the Steam client, but they won't actually unlock until you shut down the application.

This simple guide will help you on getting all the "Cores" achievements in almost any PC.
For this, you will need two things:

• Oracle VirtualBox (Freeware)
  
• Any Windows ISO (I used Windows 7 for this guide)

I won't provide any ISO download links since that would be illegal, but you can find them pretty easily in the web with a quick Google search.

After that short intro, let's start with the tutorial.

First, head to https://www.virtualbox.org/wiki/Downloads and download any of the hosts. Keep in mind that this guide is focused on Windows based systems, but still should work on other Operating Systems.
Once downloaded, install the software by clicking on Next > Next > Leave custom options enabled > Next > and proceed with the installation.
Wait for the installation, accept the 'Oracle Corporation Universal Serial Bus' prompt and run the application once it has finished.
Now, we will need to configure and install an Operating System.

Go to Tools and New.
Choose any name (I'll call it Test), a route to store the virtual machine, and the type and version. My ISO is a Windows 7 - 64bit, so it should look like this:



Click on Next, assign any quantity of RAM (I recommend 1 or 2 GB) and click on Next again.
Choose 'Create a virtual hard disk now' option and click on Create.
On the new screen, choose VDI (VirtualBox Disk Image) and click on Next. Choose 'Dynamically allocated' on the new window.
Assign any size you want, but preferably 20 GB, as you'll need to install the OS on it.
Your machine should look like this:



Right click on it > Settings > System.
Make sure to tick the "Enable I/O APIC" checkbox in the Motherboard tab and uncheck the 'Floppy' from the boot order settings.
Now, head to Network and make sure to have the following settings:

• ☑ Enable Network Adapter
  
• Attached to: NAT
  
• ☑ Cable Connected (In advanced)

Don't touch the other settings.

Lastly, click on Storage > on 'Empty' under Storage Devices > on the Disk icon on the rightmost corner and on Choose Virtual Optical Disk Drive to choose your ISO.



Look for it in your files and click on Open once you've found it. This is how your Storage Devices should look:



We're good to go, click on OK to close the settings window.
Boot the Virtual Machine by clicking on the Start Button on the top area and proceed with the installation.


Once the OS is installed, we can proceed with the next step.
Open the browser included by the OS, download any other (because IE sucks and won't let you access to Steam) and download the Steam client with your brand new Browser.
Install and run the Steam client.
Login with your account and download CPUCores. Keep in mind that it might take a few tries to successfully log in. It took me a good 5 tries to finally get in my account (but that seems to be based on games owned).
We're almost there. Shut down the machine, we need to do one more configuration before launching the software.

Head to Oracle VM VIrtualBox installation folder, usually in:

Program Files\Oracle\VirtualBox

And on the address bar type "cmd", without quotes.



This will open a command line interface at the same location.
Mine looks like this:



Once there, type
"vboxmanage modifyvm "machine name" --cpus 10" (without quotes), where machine name is the same name that you gave to your machine. In my case, it was "Test", so it would end like this:

vboxmanage modifyvm "Test" --cpus 10

If everything was done correctly, you should see a "Processors: 10" under the System section on the right side.
Boot up the Virtual Machine and verify that you have 10 cores by opening the task manager (right click on taskbar and click on Task Manager) and going to the performance tab.
This is how it should look:


(apologies for the Spanish text in my image, but for some reason my ISO was in that language).

Now, you can safely launch Steam and run the application.
This should unlock all the "Cores" achievements, but most importantly, this one:


Which seems to be the one that most people is struggling to get.

These are pretty self explanatory and shouldn't give anyone any troubles, but I'll still explain them, for the sake of a complete achievement guide.


Simply idle or use the software for 2000 hours. This can be done minimized or minimized to tray.
Additional info regarding playtime achievements: https://gtm.steamproxy.vip/app/384300/discussions/0/1484358860943358437/
Note: Playtime-based achievements will only unlock if/when your profile visibility is set to PUBLIC.


Click on Advanced Settings (on the bottom right corner), then on Theme and choose Halloween under Special Themes.

I want to thank jippen for giving me the idea for the "Cores" achievements.