Date of eml file changed

Hello,

i have a network with 5 pcs, one with win xp, the others with vista.

On the xp there are some *.eml files. When i mark such a file with one of
the vista pcs the date of the file is changed, without doing anything. how
can that be?
The problem is that we backup all files which are changed, so the old eml
files are copied again and again and that costs a lot of time.

Thanks

Martin