I don't really think your choice of browser (Mozilla) and e-mail client (Thunderbird) have anything to do with the problem you wrote about. That message you received was about the Operating System (XP or Vista) and not your browser and mail program. I would not go trying to change anything at this point.

If I am wrong, someone else please correct this information.

using firefox is very easy and you can have it on your computer right along side Internet explorer and use both at the same time. When you download and install firefox it will ask you if you want to import your favorites/bookmarks from Internet explorer just tell it yes to import everything, it will ask if you want to make it your default browser you do not have to say yes to that if you don't want to it can be done at any time quite easily and you may prefer to leave Ie as your default for now.

As far as Thunderbird goes it is an excellent email client, if you use an email address from your internet provider you can certainly set it up to get that email for you, it will also get some of the webmail email like gmail for example.

Personally I would not mess with changing over your email till you get a resolution on the issue about your copy of Windows. I would bet your copy is legit and you were one of the many that just got caught in a false positive at the check point.

Having firefox is always a good idea because you do want to have at least 2 browsers on your pc just in case IE should decide to crash you can still then get on the internet.

These are some very good links to help you get started with firefox, how to install it and how to set up and use it.

Learn Firefox

Firefox Help & Tutorials

Installing Firefox on Windows
step by step with pictures
Just be sure as you go along step by step that you do not add any of the toolbars and extra stuff they may ask if you would like to add, you can get that stuff later and you usually do not want those. Now if you want the google toolbar you can accept it up to you.

I know you did say you already have firefox on the pc but you did not mention how old it was or what version it was which is why I am giving you all the instructions on installing from scratch.

If you do have a fairly current version of firefox then you may be able to simply open it by clicking on the firefox icon on you desktop then at the top go to Help then check for updates and click that if you can get the updates that way good if your copy is old you may have to simply just do a new install which you can do right over the top of the old one. When you click on Help and about it will give you the version number of the one you have if you will let us know that version.

once we know which version you have we can then tell you how to import your favorites from IE and your settings.