1) Open the Java Control Panel.

Open up the normal Windows control panel by double clicking it's icon on the desktop (if there is one) or selecting it from the "Start" menu. Next double click on the "Java" icon. You may need to change your Control Panel to "Classic View" if you can't find it. Once you have it open the Java control panel will look something like this...

2) Open the "Java Runtime Settings" dialog box.

Click on the tab labelled "Java" and then click on the button labelled "View..." in the "Java Applet Runtime Settings" panel.

You should now see a smaller window that looks something like this...

3) Change the amount of heap space available to the Java VM.

Double click on the cell labelled "Java Runtime Parameters" (If you have more than one JRE version installed then just do this for the one with the highest revision number). Then enter "-Xmx300m" to give the Java VM 300mb of heap space. Note that just because you allow it to have 300mb doesn't mean that it will always use that much, it will use as much as it needs but will never go above 300mb of your RAM.