Greek Not Showing on Some Web Sites

[Maniote]

First, I am a PC convert.....after five PCs I just got tired of the hassels, but I loaded XP for the transition. Here is my problem: While I have my MAC MINI with 10.4.7 configured to show and type Greek -- no issue there -- when I go to many websites with Greek several of the Greek letters show as ? usually a few vowels (a good example is http://www.cretetv.gr/) About 97 percent of the page is fine but I get a heavy sprinkling of ? throughout. Admittedly, when I am operating as XP I don't have that issue, but I really don't want to access the internet anymore with the Window side. I'd just like all of the Greek letters to show. I have a friend who has an earlier MAC Mini and he has no problem with those sites. Got any ideas? (I have both Greek and polytonic Greek toggled.)