Fonts differences between screen and website

[Bastol]

Hello,

I'm French and I'm currently creating a website for a British company.
I'm working on this website since 1.5 month. I don't know why, but now, when I upload the pages on the server, with all the affiliated files, few text parts doesn't appeared like in the software.
Of course, I'm using the 'web safe' fonts (Trebuchet MS) but nothing appear like I want :
For example, if I write a text in Italic on the software, it will appear in Bold on the website, or It'll be in a huge size...
Here 2 examples :

On the software
http://img97.imageshack.us/img97/7941/37576514.jpg

On the online website
http://img13.imageshack.us/img13/2233/43362666.jpg

Another one :

On the software
http://img175.imageshack.us/img175/794/14930251.jpg

On the online website
http://img529.imageshack.us/img529/6900/53637755.jpg

I really don't understand why all is changing. I use Mozilla but I tried on IE and it's exactly the same on my PC and on another one.
I've tried to search on the properties and everywhere else without finding.
It perfectly worked a week ago, but not now.

Can you help me ?