Need a little help with a weird virus...

Early this morning, I downloaded and installed the latest (supposedly) virus definititions from Norton. I noticed that the date of release was 5 days earlier. But I swear I've updated LATER than that! Well, later on, when my daughter booted up to do some web browsing, this popped up:

Norton has detected a virus

mhtmlredir.exploit

File cannot be repaired.

Then "Ok". No query about quarantine or whatever.

So naturally, I go to their site to find out about this thing, and...it doesn't make sense. It says you cannot remove it because there is nothing there to remove!? I've installed every patch (yes, I'm completely up to date) Microsoft has issued--so, how did this creepy thing bypass the patch? Anyone know?