To launch ASDM or IDM the basic requirement is that your Java maximum heap size must not be lesser than 256mb. Adjusting the maximum heap size is easily done through JRE plugin, however this works fine for JRE version 1.4 and 1.5. For JRE 1.6, it has no effect even if you have changed the maximum heap size to 256mb.
This message appears when your Java heap size is less than 256mb, usually it is suffice to adjust from the Java plug-in directly and you should be able to launch the asdm/idm. It does not seem to work on Windows7 64-bit JRE1.6.
Most people resort to downgrade their Java plug-in to version 1.4.2_19 (the last release for the 1.4 train) and it worked. You do not need to downgrade your Java plugin anymore, this tutorial is meant to solve your current problem of unable to launch asdm/idm.
Traditional method
You go to Control Panel, and double click on the Java plugin.
You click on Java tab, and under Java Runtime Environment section you click on View button.
You add the -Xmx256m. You can also do -Xms256m -Xmx512m, but both method would not work. Your ASDM/IDM will still complain your java heap size is less than 256mb.
Method that works for Windows 7 64-bit JRE 1.6
Use Windows explorer and browse to the location where your adsm-idm launcher resides. There is a config file known as asdm-launcher.config, this file is read-only. You need to be an administrator to uncheck the read only box.
Use your favourite windows text editor and open the config file. I prefer pspad. As you can see the configuration is initial 64mb and maximum 256mb, increase these values.
Change the value as stated in the screenshot. And save the file.
cyruslab 
Thanks alooooooooot
You are welcome. Took me quite a while to figure this out. I am glad that it works for you.
thx alooootttt toooo…. hehehehe
did not understand how to fix it !
the method of adding -Xmx512m worked for me for jre-6u7-windows-i586-p