Microsoft Documents
Update: Microsoft has learned to be more friendly about file extensions. They now provide tools that will let you convert, or at least view legacy (old) file formats. If you don't find what you want, try searching. For example, search for "Office File Converter". Microsoft also plans to provide tools for the next generation of Office that will let you convert new formats to old ones.
What to do, now that you have that new computer, but you can't open your old documents because you now have new or different software? Example: The spreadsheets on your old computer were done in Excel, but your new computer only has Microsoft Works.
In this case the solution may be trivial. Try this: Click [File > Open] in the menu of the Works spreadsheet program. Then click the down arrow at the right end of the "Files of type:" line. Pick Excel SS (*.xl*) since you know that's what the old program was. Select the document (file) that you've transferred and see if it opens.
This approach is almost certain to work when you go from a less powerful to a more powerful program. In this case, it even works in the other direction. Now that you have the file open, you can save it in the new format that corresponds to the new program.
In other situations, you may need to save the file in a mutually compatible format first. Use the same "Files of type:" approach, but this time starting with the old computer. If you no longer have the old computer, find someone who still has the old program on their computer, and convert the file on theirs.
Click [File > Save As...] in the menu of the old program. Click the down arrow in the "Files of type:" line, and look for file types that the new program can probably open. For example Microsoft Word can save files as "Works 3.0 for Windows (*.wps)" or "WordPerfect 5.0 (*.doc)" or 'Rich Text File (RTF) (*.rtf). You might as well save it in all three file types before taking the next step.