Ok, I think you computer is bit confused. If you are using outlook express or ms outlook to send and receive email you will need to do this.
Are you using Microsoft Works ? I am sure something had happened there.
Anyway, Open IE 7 or 8 go to Tools, Internet Options and Programs Tab.
On Internet Programs select Set Program you will get instant access to control panel to where default programs are.
Select the last options "Set program access, and computer defaults.
You will get three profiles - Microsoft : Non-Microsoft - and Custom.
Select Custom, Click the arrows pointing down and you will get whole loads of program defaults.
On the Choose Default email program you will get a list of programs. Select them and select "enable access to this program".
This should do the trick.
However, if you use other web base email clients like yahoo or gmail. Its better you save the documents, and attach by uploading the document to yahoo or gmail.
Hope this helps.