total cost of moving from XP to Windows 8 I use Windows XP. The upgrade to Windows 8 Pro looks great value. But I will need to upgrade my Microsoft Office software as well. Is there an upgrade offer for Microsoft Office from XP to 8? If not, would I get better value by buying a new Windows 8 computer with pre-installed Office software? Or would it be cheaper overall (total cost of ownership) to change to Ubuntu or Apple?
From your question I assume you are still using Office XP? If you still have the disks for Office you will be able to re-install it after the upgrade to Windows 8 is complete. You do not have to upgrade to the new version of Office if you choose not to (I would highly recommend it if you can afford to, however, once Office 2013 is made available in a few months. It is very complementary to Windows 8 and runs much faster and is much more user friendly in my opinion.) Office XP and later should all be compatible with Windows 8.
I highly recommend upgrading to Windows 8. Overall it will provide you with the best value, and best functionality with the least effort and frustration to you.
Ubuntu Linux is the cheapest route to go if money is extremely tight and you have no other choice, however, even with Ubuntu being the most "user friendly" version of Linux there will be frustrations that come with it (such as your standard Windows software not being compatible, having to manage and download distributions, etc.)
Switching to OSX/ Apple is going to be the most expensive route out of your options. Not only are you going to have the frustrations of learning a entirely new operating system, you will have to buy into a whole new software and hardware system. Apple hardware looks nice, but it is much more expensive that comparable Windows systems, and you will have to buy an Apple system as they do not allow you to buy OSX to install on your own computer. Office does exist on a 'Mac', however you would have to buy it separately, as Windows software does not run (unless you go through the process of dual-booting or virtualizing Windows, in which case you have to purchase a full Windows license anyway.)
I apologize if any of this is either over your head, or the opposite. Without knowing your level of technical expertise it is difficult to tailor my response.