Read this and try the repair or you can download and use LibreOffice which is free.

1]: [click here

Fordy, a recent Windows 7 update has caused problems with Office Starter and it's likely that's what's happened to you. However, I'm not sure that you're right about your documents suddenly changing to DOCX files as the Starter edition can save and open those too. Anyhow, here's the fix for you to try:

Right-click on one of your affected documents -> left-click on "Open With" -> left-click "Choose Default Program". In the windows that opens, left-click once on the Microsoft Office Client Virtualization Handler option the click OK. All files of that type should now open in Office Starter.

Repeat for all other affected Office documents that fail to open as they used to. Note that for Excel files the option to select is called Microsoft Application Virtualization DDE Handler.

PS: Despite what you've been told, Office Starter does not expire. I've tackled this problem a few times in the last few days so try my advice before you do anything drastic.