If you haven't added anything, other than M$ stuff -
1. Press the "Start" button
2. find the "Default Programs" button
3. select from 4 basic settings:
Set your defauts, Associates a file type or protocol, Change autoPlay and set program access and computer default.
You will need to make a change in either Autoplay or Set Associations.
This will get you in the ball park of where things are changed. Now you need to know what kind of files are not responding or not opening.
if they are pictures, movies, videos etc. it'll be different than if they are documents. Most people know enough to send only text messages through basic mail readers, The fancy stuff, is sent through HTML. That where things can get complicatedted. By default, for security, prevent mail viruses - newer programs do not allow you to open some types of messages in mail. Unless you have anti-virus installed.
There are free anti-virus, free picture and movie readers, free spyware available on the net, most of it is as good (sometimes better) than the stuff you have to pay for.
Like it or not...death takes its tax...always!