While there's no easy way to account for everybody Leopard user's crashing, Unsanity's Application Enhancer is apparently the cause of many a BSOD turning up on a range of user's systems during a system update to Mac OS X 10.5. If, after selecting "update," you're getting a perpetual blue screen, follow Apple's instructions for booting into single-user mode to remove the offending software. If you haven't updated yet, make sure you get rid of the software before you do: alternatively, do a fresh install and it'll overwrite any of the conflicts you would encounter otherwise. And remember to backup, backup, and backup.
[Thanks, Dilan J.]