There is a chance that your system won't need to be recovered, but...you might be doing a full recover since it hung up during the OS install.....First let's get positive and go the non-destructo route
Unplug any controllers or other interface attachments, usb drives, etc.
With the system in stand by mode (red light on the power button) press and hold
the power button and the system will turn on and then turn off again (keep holding the button through this whole thing).
Now that the system is off give it the good ol' fashioned 2 second delay and then press and hold the power button. You will hear the initial beep as it begins powerup. Keep holding that power button until you hear a followup two beeps (beepbeep) and then release the power button.
At this point the PS3 will tell you to plug in a controller via the USB cable. This will bring up the recovery menu. You should see 6 menu items. You want to pick "Restart System" only at this time....anything else might get...wipe-happy with your system if it finds any bad files.
If the system is still more or less intact it'll start up at this point and you can try to run the update on it again (singing happy songs and skipping down the road).
If the system is hosed then it won't start up and you'll have to repeat the process to access the recovery menu again. This time choose "system update" and let it do its thing. Hopefully it'll stop hanging up.
If you still hang at 99%...you're going to have to restore the system which will wipe the hard drive (any unsynced trophies, all game saves not backed up elsewhere or uploaded to PSN...). Hopefully it's not this drastic. And if it is, hopefully you've got a recent backup