Well, if your emails are from a pop3 account and you have kept the originals on the server, it will re-download them

if not
1) The way I do things is have 2 hard drives, one for the OS and one for the Data.
2) The alternative is to partition the h/d and again, have one section with the OS and the other with data, therefore, if you need to re-format, you just do the "windows" section and not the actual data.
Partition magic is an excellent application if you wish to carry out step 2 which is the cheapest.
Best of luck,
G.