I had a Windows xp on my computer and minecraft sp worked perfectly.
Then I wanted to install widows 7 and when I installed minecraft and the newest Java again ,minecraft didn't want to work.I started minecraft over and over but my system didn't react at all.Then I reinstalled Java and minecraft 3 times but it didn't want to start. How to fix this?????
Try typing into your command prompt to open Minecraft -
java -jar <Location of versionnumber.jar> -Xmx1G
Should work. This will also show you a log window, so you can trace what goes wrong where. Your versionnumber.jar is located in %appdata%/.minecraft/versions/versionnumber/, and the 1G on the end means 1 gigabyte of memory to allocate.
This solved my problem. I recently re-installed windows 7 on my computer, and before Minecraft worked perfectly un-modded. After I re-installed windows 7 I downloaded Minecraft on my fresh install and started the 1.8 Launcher. When it downloaded the latest update it would complete and then get stuck just saying "Launching". I let the computer sit for more than 20 minutes and still it never got past the launcher. I opened up the profile settings and realized Minecraft couldn't open because the directory path was wrong. It didn't know what to open, so you need to change the directory path by going to the .minecraft folder which is located under your user name in the C:Drive for me.