Searched around for answers to this, found this unanswered - its exactly my problem, so ive copy and pasted it here:-

All of a sudden i can't open or save attachments that are sent to me. I am using OE6 on a win xp pro computer. Never had this problem before. When i double click the attachment to open (e.g. a pdf) nothing happens. If i right click on it and click "save as" i get an error message " The command failed to execute" If i right click it and select "save all" i get an error message " There was an error saving one or more of the attachments. Please check your free disk space and try again" Needless to say free space is not an issue
There is nothing wrong with the attachments. It does this with attachments i was able to open and read up till yesterday. I tried a new profile. Same thing.
I googled this problem and found posts from people with same problem but no answer.
Anyone have any ideas?

I have unticked the boxes in Security, I have tried turning off AV and firewall etc.

Help!