Try starting up the existing HDD in safe mode. If that boots up fine, then you might have an issue with your OS. Check to see if you recently installed any new applications or drivers. This tends to cause the OS to fail like this sometimes.
If that does not resolve your issue, open up your machine and make sure that all of your cables are connected securly; you might as well give the insides a good dusting while you are in there. I've seen instances where dust build-up around connectors caused an issue.
If that doesn't work, then I would suggest putting another HDD into the machine to see if it will boot up with the new drive. If so, then make the new drive your primary HDD and the existing drive(s) your slave HDD(s): check your jumper settings for how to do this. Once you get this set up, you can boot into the newer of the HDDs and should be able to retrive any information from the old HDD (Windows Explorer should show the slave drive(s)).
This same scenario happened to me on my new VIAO, but it was due to a physical error on the surface of my HDD. Go figure, 2 months after my warrantee expires, my 200 GB HDD fails on me. Just pray that this isn't the same issue.
Edited by Myke (11/27/06 06:37 PM)
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I swear, troubleshooting is a science: the best discoveries are always on accident...