I have an exchange acct, but also have a auto-archive PST file.

I upgraded from Lookout
(http://www.lookoutsoft.com/Lookout/download.html) to Windows Desktop
Search.

Lookout lets you specify PST files to add to the index. This is done
in the options.

Can Windows Desktop Search also do that.  I didn't see anything in the options.

Yes, it can search .pst files:
http://www.microsoft.com/windows/desktopsearch/technical/indextype.mspx
"Windows Desktop Search searches your computer for many types of
files, including documents, photos, videos, music, and Outlook and
Outlook Express items, such as e-mail messages, contacts, and
appointments."

Try looking here for the options to enable or disable:
http://www.zimbra.com/forums/showthread.php?t=3136
"select custom and then set the outlook files you want to index."

Read through this site if you still can't find the option you are after.
http://search.msn.com.my/docs/toolbar.aspx?t=MSNTbar_DS_FAQ.htm
"Note: E-mail messages will be indexed only when Outlook or Outlook
Express is running."

If it still will not work, please let me know what operating system
and version of WDS you are using and I will continue to work on this
for you.

Thanks!