Windows Mail does not support multiple identities.
About the best you can do is use message rules to separate incoming
email so that different accounts go into different inbox folders.
Another option (not free) can be obtained here:
http://www.oehelp.com/WMIDs/
A third option is to upgrade to Windows Live Mail (WLM).
Although WLM doesn't have the full-blown Identities feature
that OE had, nevertheless it has some of that functionality.
Separate folders for each account help to keep mail separated
without having to use any message rules.
Also, when a given account folder is highlighted, starting a new
message will automatically use that account identity, rather
than the default account identity that WM uses.
Gary VanderMolen