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hi I have ubuntu 8.04 and I'm trying to install windows xp with vbox but I get this error

The VirtualBox kernel driver is not accessible to the current user. Make sure that the user has write permissions for /dev/vboxdrv by adding them to the vboxusers groups. You will need to logout for the change to take effect..
VBox status code: -1909 (VERR_VM_DRIVER_NOT_ACCESSIBLE).

Result Code:
0x80004005
Component:
Console
Interface:
IConsole {d5a1cbda-f5d7-4824-9afe-d640c94c7dcf}

how can i fix this