You've got a virus. Oh noes! What do you do? Well, the first and best thing to do is to simply power down the computer without clicking anything at all, period. Just hold the power button down until your computer shuts off, then call your neighborhood Geekchick to come take care of that for you.

Sorry, My marketing friend said I should say that. While you can bail yourself out if you need to, you probably cannot do anything to make better what has just or is about to happen to you. Chances are however, that you've already screwed up and clicked on something that's got your system compromised.

That's a bummer, but it happens. And anyway, you shouldn't blame yourself.. It wasn't your fault. Your computer shouldn't have been there.You didn't want it to get infected. It wasn't acting well anyway. In the end... it probably deserved it somehow.

The worst part is that the people who write these things are maddeningly clever, and they are really, really good at tricking you into making yourself worse off than you were before, even before you realize you're doing it. It's like what would happen if say, a flu made you want to kiss everyone... with everything you do you're only making things worse. Now take heed of Aunti Jen's list of things to do. (I stole this from ZD net, but then i corrected it, so I hope they don't mind.)  

1. Understand the risks - Prepare for this inevitability. 
   1. Have a good backup plan and stick to it - if you've prepared, the worst thing about a virual infection is the downtime (see #2 below). Having all of your crucial data backed up someplace safe is imperative to keeping up with the times. Quit putting it off, and get yourself a backup plan.
   2. Obey the First Responder’s Hippocratic Oath: Do no harm. In other words, don’t make things worse. Unless you cannot get at what you need anywhere else, turn the machine off and refer to step 6 below..
2. Carry a Web-enabled smart phone and carry a big (16GB USB) stick - Pay for that data plan. Get reasonably proficient with a favorite mobile browser. Store bookmarks. Setup your phone to get your mail, and be prepared to "get by" for a few days while your situation is remedied. You might even like it.
3. DO NOT try to fix it yourself - Cleaning a virus is practically impossible. There, I said it. It's mostly luck when you manage to clean a viral infection and never have to deal with it again. More than 90% of all infections result in a re-installation of the operating system, and in my opinion, 100% should. Fixing it needs some special tools, and - especially if you don't have all your data backed up - you'll need help.
4. Find a computer pro you trust - Like with a doctor, lawyer, mechanic, plumber or hooker, a relationship with a reliable, discrete and trustworthy computer technician is something a responsible adult spends their life nurturing.  Your pro should help you with the three things above, making the one below a stress-free, no-brainer type thing. 
5. Re-install the whole schmeal - It's simple enough. If you're infected, you need to format (a.k.a. erase all the data from) your computer's hard drive and re-install that operating system from the ground up. Afterwards you'll be treated to an infection free machine that will probably run better than it did the day you unboxed it, and will certainly run better than it did before you got it all gummed up with whatever is in it. If you know how to do this, great, if not, any neighborhood shop should be able to get you back to square one for $100 or so, and most will recover your data and re-install the OS for about $250... the latter of these is what most people want, but honestly, if you've done good backups, I always suggest NOT re-dumping all the crap on your backup onto the new/freshly installed OS, but rather gather up only what you need, and leave the backup drive as a keepsake of the huge mess you've just remedied by re-installing your Operating System.
   
   BONUS:  for the studious reader who's read this far... 
6. Use a good boot-able disk from a reliable anti-virus company - this option has a 50/50 chance of cleaning your system and giving it back to you in relatively good shape after you've messed it up good. Sometimes cleaning an infestation will wreck the operating system making it unbootable, and you have no choice but reinstalling - so don't do it until you're sure you have a good backup of your data. If your infection is milder, like say you turned it off and didn't click on anything at all, chances are more like 8-9 out of 10 that something like Kaspersky's or AVG's bootable disk will fix you right up.