Before you continue, I suggest you remove the bloatware by the Control Panel and uninstalling the junk. That is faster and easier, with less potential for problems, but the steps for reinstallation are as follows: Before you do anything, backup valuable data, if any, on some form of media (i.e. USB) so you can move it onto the reinstalled Windows. Download the appropriate ISO file and burn it to a CD or use it with a bootable USB. (Optional if you have no internet) Download all the appropriate drivers for your machine and save them on media for later access. Locate the product key on your machine. It is usually under the laptop on a sticker, and if not, use special software to locate it.
This will be used during the reinstallation. Note that PCs sometimes auto-activate if it is in the BIOS so this step is not needed if this is case. Boot from the bootable media (disc or USB). Continue as usual through the installation, and when the partitions screen comes up, format the current Windows partition and install in there. (The Recovery partition can be deleted, but I recommend keeping it so if anything goes wrong you can restore to factory settings). If the key doesn't work you may need to contact Microsoft and activate over phone.
After installation, install the drivers and run Windows Update and you're done.
Now the fun part. Install Windows 8.x be sure to pick the right edition that came with your pc or laptop. Download & Run Windows 8 Product Key Viewer 1.4.7d from Softpedia. Check the MSDM KEY checkbox, and copy the key. Then type changepk & Enter. Paste in the key. Windows 10 upgrade. Most initial installations of Windows 10 are currently from an upgrade from a previous version of Windows though Windows update. However, like other versions of Windows, it can also be installed from an ISO file.