It should really open that way by default, but there a one or two
exceptions I've come across. The first issue I had was where the commas
were not the expected ascii value. If this is the case then you need to
replace all the commas with the right ones.

Another possible reason is that excel is actually set to open them that
way, in one of the options menus. This might have been done by another
user. These menus vary by version, but you can read about them by
looking at the help file, under importing data.