How do I remove JS:Nimda-B [Trj]?

No, Malwarebytes Anti-Malware for Mac won't detect that, because it's not Mac malware. That's a malicious JavaScript, probably found in your browser's cache. It was probably found on some site that you visited, and was already run in your browser, leaving your Mac no worse for it. It may have done something like display a scam pop-up or something similar, or may have attempted to infect your machine via Windows vulnerabilities (which would not work on your Mac).

You can safely ignore this. If you clear your browser's caches, that should make it go away.

I'm having the same issue, as apparently are a lot of other AVG users if their support pages are anything to go by. I've cleared cache and history in Safari as suggested by thomas_r, but the problem remains. Any other suggestions please? I'm getting desperate and nervous about using my Air until this is sorted.

Ignore it. There's no reason to be concerned, as this is not Mac malware.

From some other reports I've seen in the last 24 hours, involving AVG and JS:Nimda-B, I believe this may be a false positive (ie, an erroneous detection of a legit file as malicious).

I used Finder (on All My Files) and cmd+shift+G and typed in the location of the trojan (not including the long number) given by the AVG scan, and then manually deleted the infected file. It involved typing in my password to delete it so I assume thats why AVG struggles with it.

threat by trojan.JS.Iframe.BKD on MacBookPro

more

https://www.google.dk/search?q=osx+remove+JS%3ANimda-B+%5BTrj%5D&ie=&oe=#spf=1

Try this...

https://www.malwarebytes.com/mac/

Indeed, I ran through with AVG support and all I had to do was reset my internet browsers (in addition to clear history), I did a scan and it then disappeared.