1. Go to Control Panel
2. Find the JAVA icon, and click on it
3. Java Control Panel will open and you'll find 5 or more tabs, select Java tab
4.- Push the "View..." button for Configure Java Runtime Environment.
5 .- Here is the trick, java uses only 256 mb of memory for his own purposes, and Minecraft needs more than that.

See how much ram you have and increase java's memory usage as this:

Guru's says that you can let java use the third of your total installed memory, for me its easy as I only have 3gb of installed ram on my computer, so i'm gonna let java deal with 1024 mb of ram (1 Gig) how?

add this command in your Runtime Parameters on each entry that you have in this list.

-Xmx1024m

as shown above no spaces with the minus symbol at the beginning and an "m" at the end.

in the above example, 1024 is the amount of memory that java will use, you can try 512 for small machines or 2048 for big ones.