Since this seems to be affecting a lot of people, I thought I would throw in some of my knowledge and possibly help someone else, or at the very least keep it from happening to them.
As some of you have noticed, LJ's full-screen ads are fairly loaded with viruses. Yes LJ is aware of this, and no the wonderful darlings are not doing anything to resolve the issue. A few ads in particular are slipping in an IMPOSSIBLE trojan that has completely killed the computers of several people in my RP community alone, including myself. A firewall, you say? Good antivirus software? MANUAL REMOVAL? All completely useless. I quite literally tried everything under the sun, and it still murdered my dear laptop. And I promise you, my friends, I am no idiot when it comes to computers, nor are the three or four other computer professionals I had trying to help me.
Hopeless? Not quite. There is a very easy way to protect yourself from these ads. The full-page LJ ads work with something similar to javascript, so it's as simple as enabling a script blocker in your browser. Script blockers
NoScript is the Firefox add-on I use personally, which is a very nice software, as it has a menu to show you which scripts are running on a particular page, and allows you to disable certain blocks as you see fit. (In other words, you can enable scripts that are essential to LJ function, but disable the scripts that make those full-screen ads pop up.) You may also want to check your particular antivirus software, as some have script blockers built in.
If you do not use Firefox, and your antivirus doesn't have script blockers? Google "script blocker software" to find a program which will work best for you.
TL;DR version: LJ ads kill your computer deader than dead. Use script blockers to stop it.