On my WinXP Home laptop, IE and Firefox live side by side very easily. You just click on IE or Firefox to start the one you want. If you are using some sort of Dail-up management software--it may start IE automatically until you change its settings.
There is a default setting in IE and Firefox that allows you to make one or the other default.... For example, if you have been using Firefox as your default, clicking IE will bring up a "Change your default to IE?" You can click yes or no. If you click no, you can still use IE just fine. It is just if you open a link from(for example) an email, your default browser will open instead.
The only "difficulty" is that Firefox will import all of your existing IE bookmarks. From this point forward, any new book marks will exist only in the browser where you created it. --that is about the only issue I have seen when running both browsers at the same time.
Also, you may wish to think about using Thunderbird as your email program. It will import all of your addresses and email from Outlook or Outlook Express.
Both Firefox and Thunderbird are much less likely to be infected than their Microsoft equivalents.
I hope I was clear--It is really pretty easy and straight forward.
-Bill