because many programs (such as antivirus and Adobe products) do not work !
Compatibility is up to the program maker, they need to release an updated version.
Since Windows 8 has not been released yet not all programs are going to work with it. Some are very specific about the operating system version, so you might have to wait until they release Windows 8 versions.
If the programs don't install, try installing them in compatibility mode.
This can help with some (but not all) programs/drivers:
1) Uninstall the program if it's already installed. Then when you install/reinstall it instead of double clicking on the install file right click it and select "Troubleshoot Compatibility"
2) Then click "Try Recommended Settings" and in the next windows click "Start the Program" to install it.
3) After it installs see if it works correctly. Please note - some programs may not be able to run in Windows 8.
I was using Windows 7, and, unintentionally, I installed Windows Developer Preview over it,
You should not install Windows 8 Developer Preview on your main computer - it's not even a beta product yet and bugs are to be expected.
From the main download page:
The Windows Developer Preview is a pre-beta version of Windows 8 for developers. These downloads include prerelease software that may change without notice. The software is provided as is, and you bear the risk of using it. It may not be stable, operate correctly or work the way the final version of the software will. It should not be used in a production environment.
Also, it is stated on the Windows Developer Preview download page that you must reinstall the previous OS:
Notes about installing the Windows Developer Preview
You can't uninstall the Windows Developer Preview. To reinstall your previous operating system, you must have restore or installation media.
ReinstallingIf you have install disks - insert them, restart and boot from them to start the reinstall process. Reinstalling will remove any personal files on the computer, back them up first.
If Windows came with the computer - Reinstall Windows Preinstalled On Computer
OEM computer manufacturers are required to provide at least one way for you to reinstall Windows if you buy a computer with Windows preinstalled - your computer should have included either a restore partition on your hard drive, or Windows install/restore disks. Most computer manufacturers also have a feature to make your own install disks.
If the restore partition is still there and was not removed, it will reinstall Windows 7. There is usually a prompt when you first start the computer telling you what key to press to start the restore - check your manual if you're not sure how to. If you have install disks - insert them, restart and boot from them to start the reinstall process. Reinstalling will remove any personal files on the computer, back them up first.
If you don't have the restore partition or install disks:
Contact the OEM computer manufacturer and ask them to send you an install disk for your computer - they may or may not do it. If they agree to send you one they may charge a small fee.