If you want to re-install you can download the netinst from debian.org. It contains only the files necessary to start up the installation, you can then select only what you want to install. You'll need to be able to boot something up on the virtual machine, and have admin privileges.
If you want to uninstall applications you consider unnecessary you'll need root privileges. If there is a graphical environment just go to
System > Administration > Software Center, select
Installed Software and uninstall whatever you don't need - be careful not to remove anything that is needed in order for the VM to work.
If there isn't a graphical environment you can issue
aptitude search '~i' to get a list of packages installed (the ones automatically installed have an
A before the name). Then
aptitude remove package_name to remove the packages you don't want - again, all super user (root).