that windows doesn't know what programme you want to use to open the attachment.
Generally this is for one of two reasons;
1)You don't have a suitable programme installed,
2)You do have a suitable program installed, but Windows doesn't know that you want to use that program
First of all, what type of file is the attachment?
That is to say, what is the extension at the end of the name? Is it .jpg perhaps, or .xls or .pps
We can help more with this information.
(But I'm afraid it's time for bed for me! I've to be up in just a few hours!)
There are loads of folk on here who will be able to help with this.. don't worry!