Hi Phyllis,
Do you have virus protection software like McAfee or Norton? Also, is the software up-to-date? I don't mean to worry you, but since this problem is affecting all the office products (Word, Excel, and Power Point), you may have a virus. I'm assuming that your not having problems opening all files, not just one file. There has been recent news of a "zero-day" attack, etc.

I would do a complete scan of your hard drive as soon as possible. Some of the new worm/viruses won't let you access your virus software or it won't be detected if your software is not up-to-date, so if you have trouble running the virus protection software, then you should take your computer in to get looked at by a pro. Always better to be safe than sorry.

Good luck,
Greg