help!!! my mac will not start after aborted security update

hi all, i hope you can help - I'm absolutelty up the creek without a paddle!!

Earlier today i decided to use an external hard drive as a Time Machine drive to make a 'proper' backup of my iMac (I had just been manually copying files over to it every so often, but thought....well...the pubs are all closed today, not much to do, so I think I'll finally get round to making a real backup using the Time Machine utility).

but to do that, i had to format the drive, and so until the Time Machine backup completed, I was in that awful position where i had no backup of my Mac.

Of course, sods law....the backup failed (no idea why, just some stupid non-descriptive message in the backup window with no details as to what might have happened). While waiting for the next scheduled attempt, I decided to delete whatever was on the external disk (i.e. the failed backup), and just by chance then I got a software update pop-up on the Mac...

iTunes, and Safari I think, along with a "Security Update". I went to install the updates, but one failed, and now I am unable to start up the Mac at all.

all I have is the white screen at startup with the apple logo, then I get a kind of 'forbidden' or 'no entry' graphic, and I can't do anything with the computer.

anyone know what I can do? I can't use the restore discs because i'll lose EVERYTHING, and because I deleted the contents of my external drive to use the Time Machine to make a 'proper' backup, I am really screwed now.

My guess is that the files are all still there on the disk, I just can't access them. Would I need to bring the computer to a specialist, or is there some trick I can do to get actually on to the OS? even command line access would be fine, as I have unix experience from my day job, so I could mount a DVD and manually copy over the files on the disc then and do a restore to factory default then.

any ideas? would really appreciate any help you might be able to offer.

thanks in advance,
andy

Which version of OS X are you using? If it's Leopard, you can do an Archive and Install with your recovery DVD and not lose anything. If you have Snow Leopard, reinstalling will not remove your data and documents but will overwrite system files. You would then have to download and install all updates again.

You might also be able to start up in Safe Boot Mode and remove whatever security update or update that's preventing the machine from starting.

Hi! sorry for not getting back to you - just wanted to say thanks for your help, i ran the Utilities from the Install Disk but unfortunately I wasn't able to save all of my file, and ended up losing some photos and AVI files that seem to have become corrupt for whatever reason.
I had all my music on my mp3 players, so nothing lost there thankfully, but some of the video footage is gone now, as that was only on the mac

Even after i ended up doing a re-install of MacOS, it still behaved strangly (freezing up completely (apart from being able to move the mouse), and then suddenly it would act perfecly normally and I coul copy files etc from the mp3 players, but then it'd freeze again and so on an on...

ended up leaving into a Mac specialist centre in Dublin as I haven't got the experience needed to sort out whatever might be wrong.

thanks again!
andy