Just press escape during the grub loading process and choose the second kernel :), that's it! If you want it to be persistent to avoid doing that at every startup, just go edit the "/boot/grub/grub.conf" and change "default=0" to "default=1" to point to the second kernel automatically.
Not able to install Guest Additions:
- mount the installation DVD, navigate to the folder "Server" and issue the command "rpm -ivh kernel-devel-2.6.18-164.el5.i686.rpm"
- unmount the DVD (if you're not able to unmount due to a bug in virtualbox that prevents unmounting, just shutdown the guest OS and then remove it from the storage section)
- In the guest OS, click "Devices"->"Install Guest Additions"
- The CD will be mounted, navigate to its folder (in /media/VBox***) and run "sh VBoxLinux***x86.sh" (x86 here if you're using 32-bit edition)
- Reboot and enjoy :)