Not because it is new and different, although that will be a problem. There was nothing wrong with Windows Vista, but people hated it, mostly because it was different from XP. The jump from Windows 7 to Windows 8 is even bigger. People will not be able to find their way around it. They will get confused and annoyed.

Not because it is ugly, though that will be a problem. If you really want big clunky icons on a boring background, Windows 7 will let you do it. So giving us big clunky icons splattered all over a boring background and telling us this is the exciting new Metro interface is not going to convince anyone.

Not because the controls are confusing, although that will be a problem. How do you close a programme? How do you turn the computer off? How do you check for updates? All these things can be done, but not in any obvious or intuitive way.

Not because the Metro apps are slow to load and hard to configure, although that will be a problem.

No, the real reason Windows 8 will fail is because it hinders productivity at every turn.

As I type this, I have three Internet Explorer tabs open, plus Outlook, Notepad and Word. I can see every open programme on the taskbar, I can change between them with a single click, I can copy and paste between them with a few keyboard shortcuts. The ability to do this is essential to my workflow, as it is for everyone who works in business. There is no easy or obvious way to do this in Windows 8. You cannot easily see what programmes are running, you cannot easily move from one to another, you cannot easily transfer data between them.

I am not saying there is no way to do these things, just that there is no easy or obvious way to do them. This is a major drawback compared with every version of Windows since XP.

After using the Windows 8 preview and beta for the last several months on my home computer, I could not wait to get back to Windows 7. Windows 8 was a slog from start to finish.

Windows 8 might be suitable for tablet PCs, although the metro interface offers little reason to choose it over Android or Apple OS. But tablet PCs are a fad. They are no more than pretty toys. Even if you want a highly portable computer for simple tasks like email and internet, in almost every circumstance you will be better off with a netbook with a proper built-in keyboard.

A tablet PC is the only place Windows 8 might work. But it is incredibly stupid to design a whole new version of Windows for a type of computer that will never be more than a tiny proportion of all PCs.

For normal home or business use Windows 8 is frustrating, verging on hopeless.

Businesses will only invest in a Windows upgrade if it will improve workflow and productivity. Windows 8 does the opposite. No one will want it, and I will be embarrassed to sell it.