Sounds like corrupt files (via bad hdd / corrupt filesystem).
Start with the basics. Open word on its own (type 'word' in the Search section of start menu). Does that work? Download a .doc and .docx file from the internet (should't be difficult to find). Note, download the files (right-click, save as if necessary)...once they are downloaded to your desktop, browse to the file and open from word or from Windows explorer. This will ensure the problem doesn't stem from opening via web-browser plug-ins, which can be problematic.
See if you can email a file to a friend. Can THEY open the old files?