I have recently upgraded some of my websited from html 4 to xhtml transitional with the html 4.01 I could use target="_new" in the href's and the link would open in a new window.
The alternative is to not force your perception of functionality of a website upon other users. The intention behind removing the target attribute from the specifications (even in (strict) HTML 4, by the way) was that the users should be able to decide for themselves whether or not they want to open links in new windows. If you still want to enforce this you should use JavaScript (hint: window.open()).