Due to your DNN version, you will need to look to 3rd party vendors for the Url Rewriting functionality, or just try adding the UrlRewriter httpmodule from a newer DNN install.  However, the results of adding it (in bin, and web.config) may not be as expected.  Since your version is so far back, you may need to write your own UrlRewriter based on the current DNN one.  (It is not as hard as it sounds - mostly copy and paste, then fix compile errors until they are gone.)

The regex in newer DNN installs will work.  The RegEx looks for the URL matching anything that has "/DNN/" in the path.  However, it should be ammended to optionally look for the trailing slash, and the replace should include the trailing slash.

(.*)/DNN/*(.*)
$1/$2

That will catch any URL with the DNN path in it, and using the groups property of the regex class, recreates the same URL with the $1 (second) and $2 (third) groups.  So to explain further, here is your match goups:

$0 = http://www.domain.com/DNN/Home/SomePage/tabid/123/Default.aspx
$1 = http://www.domain.com
$2 = Home/SomePage/tabid/123/Default.aspx
$1/$2 = http://www.domain.com/Home/SomePage/tabid/123/Default.aspx