There are a lot of commonly occurring issues appearing in the Installation sub-forum, so please take a look through the list here and see if these can solve your problem before creating a new one!
For a Upgrade installation guide > Windows 10 - Upgrade Installation - Windows 10 Forums
For a clean installation guide > Windows 10 - Clean Install - Windows 10 Forums
For Activating Windows 10 > Activate Windows 10 - Windows 10 Forums
Multiple Installation failures / Error 0x80240020 - This appears to be caused by corrupted / missing files in the pre-download for Windows 10. Following these steps will clear that cache, then re-download the files.
If repairing the files do not work, please try following the media creation guide on Windows 10 - Upgrade Installation - Windows 10 Forums to upgrade from an ISO / USB.1) Access this file location C:\Windows\SoftwareDistribution\Download and delete everything there. (Note delete only content found there and not the Download folder itself)
2) Open CMD (command prompt) with run as administrator (admin privileges) and run this command: wuauclt.exe /updatenow
Taken from > upgrade to windows 10 pro failed - Windows 10 Forums
Error 0x80070070 - This is a general out of space error, meaning you do not have enough space on your C:\ to install the upgrade.
Uninstalling / removing applications or files and allowing at least 30Gb of space should remove this error.
Taken from > We Couldn't install windows 10 0x80070070 - 0x2000C - Windows 10 Forums
Error 0x8007002C-4000D - This appears to be an issue with upgrading a system which has Symbolic links that do not resolve correctly, or applications which have redundant links.
These can be found by navigating to - "%WINDIR%\WindowsUpdate.log" and reviewing the logged information that contains the file that causes the error and removing it.
This was also an issue from the installation of Windows 8 via Upgrading.
Activation issues with Clean Installation of Windows 10 - The short answer is that you need to activate Windows 10, through the upgrade process first, before you perform a clean install.
Please see @Brinks post for the information regarding this > Clean Install after upgrade from 7 don't work !! Can't activate the OS - Windows 10 Forums
Blank Black Screen After Installation - This appears to be caused by default video settings within windows when the installation completes and It appears to be occurring within Laptops primarily.
The fix, is to use the Windows + P buttons to navigate through the Display selection windows and selecting the correct display adaptor. (Trial and error if you cannot see which selection it is.)
Thanks to @klemenhe for pointing this out > Windows 10 install stuck at black screen without cursor or anything - Page 3 - Windows 10 Forums
If you encounter errors whilst running Windows 10, you can access the recovery menu by following these steps.
1) Press Start and bring up the power options.
2) Hold SHIFT whilst pressing Restart.
The computer will restart and show the blue recovery menu(The same as Windows 8)3) When the machine reboots, click Troubleshooting
Lose of Rights / Admin access required for certain files / folders.
During the upgrade / installation, your user account is changed and may lose access to files stored on separate drives or areas.
To remedy this, you will need to 'Reset' the ownership of those files, which can be done by following this guide > Take Ownership - Add to Context Menu in Windows 10 - Windows 10 Forums
Change User Profile Name
For some reason, during a clean installation, your user profile will use a generated ID, rather than that specified. This can be seen here > Solved User Profile Name Change - Windows 10 Forums
The fix for this is the same as with previous versions, by using this guide on SevenForums > User Profile Folder - Change User Account Folder Name - Windows 7 Help Forums
I feel that more will be added as we go through this, so please bear with us!
If it is not in the list, or you still have problems with a previously mentioned item, please create a new thread and highlight what steps you have already followed, and the outcome!