Hi, Sam -
Here's a link to the user guide for that router -
ftp://ftp.linksys.com/pdf/befw11s4_v4_ug.pdf
To directly answer your question, I'm afraid there isn't
anything in your router that'll let you block certain
sites. About all you can do is block certain *computers*
from accessing the Internet at all.
If you're trying to block sites based on content there
are tools within Windows XP you can use (take a look at
Content Advisor within Internet Explorer) or you can
purchase a third-party program to do the job if you like.
Hope this helps -