I've had this problem many times with my ps3, however it is quite easy to overcome...
The reason the update is stopping at 99% is because of a damaged or corrupted hardrive (HDD).
Don't Panic! It's easy to fix.
Below where you turn your ps3 on you will have to locate the hardrive casing, take it off, and pull the hardrive out. It will be a little silver box. Clean it with either a cloth or a tea towel, then put it in a room and leave it for a day. Put the hardrive back in, and it should load to 100% in one go, or it will come up asking for a new HDMI source, which is good.
If this fails then you can try putting your ps3 into safe mode:
1. Make sure your ps3 is turned off. (I know it says not to turn it off but it's the only way to fix it.)
Hold the off button for about 10 seconds if it won't let your turn it off.
2. Hold to power back on, hear the first beep, do not let go of power button, until you hear the 3rd beep. This will again power off the ps3 and it will now be in safe mode.
3. Power on as normal.
I hope this helps and at last resort I would e-mail the sony support helpline at: firstname.lastname@example.org