I'd simply go to Microsoft (or cnet or "file hippo" and download and install a fresh copy of Explorer8, but there is no reason whatsoever you can only have one browser on your computer.
you can only have one default browser (choose carefully)
I'm not in the microsoft hate society as some are, but I've never
been happy with Firefox.
I've got Google Chrome but I've only spent a few hours playing with it.
So I can't really judge...
As for your virus problem...
What Virus protection programs are you using?
IF you are running a windows OS (you probably are because the other OS's are sterile mules) check your Security status, check to make sure you are getting automatic updates (most deal with "security") from Microsoft, but go to the windows update website and click the "custom"
I use both the free version of AVG and Microsoft Security Essentials(free download from Microsoft) along with Windows Defender (also free)
but I don't depend on "automatic updates" to keep the definitions updated I go to the site a couple times a week and "force" the updates..
Even most of the optional ones....
But if you manually select updates on the windows update website one of them will be Explorer 8.
I got Explorer 8 on several systems I've set up for others in the last couple weeks by simply letting automatic updates do it's "thing" automatically, because updating 6.x or 7.x to 8 is virtually automatic unless you have automatic updates turned off.
I had intended to update to 8 manually, but it saved me the trouble
(I have a 4gb flash drive that's loaded with all my general use drivers
and "install kernels" for what I consider "necissary" software. on freshly set up machines)
I don't make fun of IT guys anymore because I've lately come to the belated realization that I are one:)