one way to do this is (but not the most convenient way)...
Create a new .ASP page called start.asp and put the following in it:
<% response.redirect("http://mysite/store/default.aspx") %>
(Or the asp.net alternative) If you do not have classic ASP, then create start.htm with :
<meta http-equiv="refresh" content="0;url=http://mysite/store/default.aspx" />
And simply set up either start.asp, start.htm or whatever you want as the new default document.
Anyone who vists http://mysite will be redirected to http://mysite/store
Whilst this may not be the best way, if you want to redirect everyone from the main site to the store/default, this should work fine. To get back to the main application, you just need to type an absoloute path of the existing file such as http://mysite/default.htm
If you want anything more advanced, you may want to take a look at URL Rewriting (I am sure there is a better link to a ready to download module, but I can't find it.)... but this may be over kill for what you need.