Sounds like the executable has messed up permissions, meaning you're not allowed, for some reason, to execute the application.
My friend made a little app to fix it:
http://www.versiontracker.com/dyn/moreinfo/macosx/21794
I can't remember if it still works in Tiger or not, and I don't have a broken app to fix. Come to think of it, it didn't work last time I tried it, so you should probably do this instead: (this is what the app does automatically)
1)Right Click (or control click), the Preview application in your applications folder.
2)Go to "Show Package Contents"
3)In the folder that opens, open the folder "Contents", and in that folder "MacOS" In that folder there should be a black rectangular icon entitled "Preview. Leave it alone.
4)In the "Utilities" folder in your Applications folder, open the app called "Terminal".
5)In the terminal window that comes up, type, without the quotes, "sudo chmod +x " (with the space on the end)
6)Drag the black "Preview" icon from the "MacOS" folder into the terminal window and drop it there. It should add a line of text to the end of what you just typed looking something like this: "/Applications/Preview.app/Contents/MacOS/Preview "
7)Press enter, put in your admin password, make sure it doesn't give you any errors, and then try to open the Preview application in your Applications folder.
If that doesn't work, why not give that app I mentioned earlier a shot...might as well.