Installation on Vista 64bit

I have several computers running either XP or Vista (64 bit). If I attempt on the Vista machine to attach a 760 kb and a 1.9 meg file they initially load in reduced size, as expected. When I then select the option to return to the original size the smaller file succeeds but the larger one returns with the message '1 file(s) can't be sent'. This happens equally with both IE8 and Firefox 3.5. However, when I perform the same exercise on the XP machine it seems to work properly. Is there an issue with this version of Vista?