If you want other users to be able to use it you need to add the global system environment variable
VBOX_USER_HOME=C:\VBOX (or where everyone can get to it, Document and Setting\myprofile should not be accessible by anyone else.)
You make a global system environment variable by doing a right click on My Computer and select Properties - then select the Advanced tab - then the Environment button near the bottom then the new button under Systems Variables the variable name is VBOX_USER_HOME and the variable value is the path like C:\VBOX.
copy the .virtualbox contents from you myprofile to the C:\VBOX (or what you called it) and reboot and everyone can see the Virtual Machines. At that point you can remove the contents of your myprofile\.virtualbox directory ( I left it incase some Vbox upgrade needs it).