I like opera (since you can close the browser and it when you reopen it, it can remember all the tabs you had open) on the other hand it's not free.
Between the two (ie and firefox) it's not even close....firefox hands down.
I've been using it regularly since 0.9 and have been very happy with it. It'll even import all your bookmarks and passwords upon setup.
And tabbed browsing (which opera also has) is the shiznit.
You may occasionally (1% depending on what you use it for) need to open IE again for certain sites (windows update for example).
Forgot to add that another important security precaution is to STOP using Outlook (aka Lookout Express). I've been using Eudora for about 5 years and haven't had to worry about most of these mail exploits.
By taking these two simple measures (Firefox and Eudora) you will solve about 95% of your security concerns....unless you're into obviously dumb things like opening unknown files that people send you.