Ubuntu and safe web surfing?

[Scott Ehrlich]

My sister's PC was gifted to her with pre-installed Windows XP.    She
thinks it has become infected, and she has very limited funding, so I
said either buy a copy of Windows XP or Win 7, though I'm not
convinced the system can take Win 7 and Win XP is being phased out.
Therefore, I would recommend Ubuntu, and 10.04 is around the corner.

She has a 10 year old son who is getting more interested in web sites.
  What options for web filtering exist and are easy to use and manage
to protect her son, my nephew, from accessing improper sites?

She lives far enough away that her husband and she would have to do
most all the management and neither is technically adept - something
comes up, they either throw out the PC or call me.

Thanks for any insight/help.    I just can't think of any easy answer
to Linux web content/page/url  filtering/management.   Maybe one of
your answers will be something I simply lost track of...

Scott