There is an Adobe support article that talks about installing the PSE5 on an XP computer then copying the serial number key from that computer's registry to your Vista computer's registry. Does any of this fit your situation?
Invalid serial number error when launching Photoshop Elements 5.0.2 on Windows Vista
To accomplish what the tech note talks about you'd need your serial number and the install media and an XP computer to install on.
If the last time you used PSE5 successfully is only a few days ago, then you can try doing a System Restore back to a restore point just earlier than that and maybe things will be back to normal.
Did you run a registry cleaner, or other fix-my-pc-one-click sort of thing, that might have removed this important PSE5 information from the registry? If so then do a system-restore back to just before you cleaned the registry.