Which version of VBox did you have installed? VBox 4, or an earlier version?
You don't upgrade kernels, you install new kernels (along with source headers). Previous kernels remain on your system until you decide to remove them.
After installing a new kernel, and if you want to use it right away, on the next re-boot selecting the new kernel from the boot menu should result in the dynamic module system building an necessary modules for the new kernel, things like video drivers, and virtual box. Vbox should run as long as the new kernel module was successfully built on reboot. If you use a graphical boot then you may not have seen the messages.
So exactly how/what did you install and what exactly did you do after installing the new kernel?
You could check in synaptic that you have the kernel-devel-2.6.38.2-pclos1.bfs package installed along with kernel 2.6.38.2-pclos1.bfs.