G4 Problem - Blue Screen following start up

I have a G4 which will not boot up following an attempt to download a recent software upgrade (Security Upgrade 2008 - 002). Upon restart the G4 now only shows a "Blue Screen". I attempted to resolve the problem by pressing "shift" after hearing the start up sound and releasing after I saw the Apple and the pinwheel but that has not resolved the problem.
Any suggestions?
Thanks, SFB8

*Moved thread to appropriate forum - This is not a FAQ Submission*

G'day and welcome to the Forums.

Take it you are using Leopard as your post does not say? Boot from the install DVD, go to Utilities and run Repair Disk and then Repair Permissions. Go have a coffee as permissions can take up to 25 minutes.

If you are experiencing update problems, do not use Software Update but go to the Apple web site and download (only) from there. When completed, run Repair Permissions, disable antivirus if used, disconnect printers etc, everything but the keyboard and mouse, drag download from the Downloads folder to the desktop, double click and away you go.

When finished run the Repair Permissions thingo again.

Let's know how you go?