One way of making the links  or the documents to open in a new window  is by using FrontPage.

Open the page in FrontPage.
Right click on the document link and select Convert to XSLT Data View. Then again right click on the link and select Hyperlink properties and set target frame to new window.
You will have to do this only for one document link other links will automatically open in a new window.
But remember editing a page in FrontPage will make  the page un-ghosted, which you don't want.

Another way is writing a custom web part using OM.

Check out Todd Bleeker’s article on doing this here:

mindsharpblogs.com/.../654.aspx

No FP editing or custom web part needed. Just use the CEWP

All the solutions so far are toward to links  in a list web part, not for the links in portal  listings that I asked …

During my search, I found a very easy way to enable links in a list web part to open a new window  and would like to share it in here.

Replace the attached schema.xml file to the C:\Program Files\Common Files\Microsoft Shared\web server extensions\60\TEMPLATE\1033\xxx\LISTS\ FAVORITE\schema.xml. (make a copy to the old schema.xml before you do this)

The xxx can be any folder you would like the lists to be window targeted. For instance, for portal area, it would be ……\60\TEMPLATE\1033\SPS\LISTS\FAVORITE\schema.xml.

You may need to reset IIS to make  this taking effect. If anything works fine, when you add a new link to a list web part, you have an option to select new window open target.

Did you check out my blog post on Opening document in a new window  using
JavaScript? I'm sure that this approach could be used on Portal Listing.

Also, I believe that Andrew Connell (http://AndrewConnell.com/blog)
posted a way to do this using the schema.XML. I wouldn't replace the
entire file and I wouldn't update Microsoft owned site defintions.

I'm not sure if you can edit the URL on these links, as I don't have Portal running at the moment. If you can, just add a target="_blank" attribute to each of your anchor tags.

Example: <a href="newpage.htm" title="ALT Text goes here" target="_blank">This is a hyperlink</a>

This will load newpage.htm in a new window.