attacment in O Look & OLexpress

cant open attachment received in outlook or outlook express. some times i can see the logo of the attachment like acrobat, word, power point and some times cant, (only can see that there is attachment but cant say what type is it.) cliking on the attachment will not open the attachment. if i do copy and past of the attchment i can open the documents. also most of the times if i select reply to the massege i can open the attchment from the reply window
OLexpress ver6 OutLook 2000. running antivirus AVG then uninstall & installed Mcafee2006
thank you for your help