I've had this launcher for over a year, no problems. It's the regular Minecraft.exe launcher you can find on minecraft.net. All of a sudden, when I opened it, Avast said it blocked a malware site. The URL it blocked was:
http://pixel.quantserve.com/pixel/p-19UtqE8ngoZbM.gif
I do not recommend clicking it if it was changed into a link. I searched "pixel.quantserve.com/pixel/p-19UtqE8ngoZbM.gif" using Google and found it occurred in the HTML code of many sites that said they were being attacked, or sites that are about hacking.
Why has Avast suddenly blocked this? Did my launcher change, or was it a virus just discovered? Is it an actual virus?
EDIT: ANSWER FOUND:
I found out why. Apparently, it is a problem. Tumblr has been hacked. The image is set to appear at the end of every page. When you go to tumblr.com, it gives you a internal server error. EDIT: It does not say internal server error anymore, but the image is still at the bottom of every page. At least, every page the Minecraft launcher loads. The Minecraft launcher displayes something from tumblr, and tumblr has a img tag requesting the malware url, which, is actually malware. See http://forum.avast.com/index.php?topic=114593.0 for more information.
Best Answer
Calm down, tumblr.com possibly had some configuration error that caused a regular website to be loaded instead of a picture. First, have you looked up what quantserve exactly is? It is just a web analytics tool, not malware (statistics page here). Even Stackoverflow uses it.
As for the Google results, it seems that Tumblr generates HTML that can be put on a website. Users then copy it without making modifications, resulting in a tracker being inserted. Example on SO.
Calm down. Really, this is just noise, there is nothing wrong with this. Avast just scared you. If you do not like it, use a Linux distribution (say, Ubuntu) with Firefox+NoScript.