I thought I would create this thread to provide information on how to fix virtual box guest additions when performing an upgrade from BT5 > BT5 R1 using apt-get dist-upgrade. This is not my own work but was found be searching on google but thought it might help having reference here. I am not sure if this effects everyone but it certainly did me and one other.
Basically when upgrading to R1 the kernel is updated breaking guest additions due to the kernel modules not being present for the new one (in this case 18.104.22.168). I carried out the process of reinstalling however this failed displaying an issue due to the install process not being able to find the kernel headers.
If you are also experiencing this issue follow the link here to view a ticket logged at the vitual box site #9456
But to summerise:
1. apt-get install linux-headers-22.214.171.124
2. cd /lib/modules/126.96.36.199; ln -s /usr/src/linux-headers-188.8.131.52 build
3. /etc/init.d/vboxadd setup
Dont forget to restart