Ugh. This may be an isolated problem, but let's assume for a moment that there could be serious problems. This would be a very good time to make a fresh backup of your documents, projects, etc.
The Disk Utility app should be in the same folder as Installer. If it's also showing a broken icon like installer has, you can boot from the Os X install disc and run it from there.
From Disk Utility, try both Verify Disk (bottom right) and then Verify Permissions (bottom left) against your boot volume, to see if any problems are reported. Go ahead and do the repairs (to repair the disk, if necessary, you will want to boot from that install disc as above, or you won't be allowed to fix).
If problems are found and successfully repaired, see if that took care of installer.
If there were no problems found in Disk Utiility (or they are repaired now) and installer is still broken, you may simply need to replace the installer app with a good copy. You can try copying the Installer.app from the Os X install disc as a quick fix.
If you're not confident that Installer.app is an isolated problem at this point, you may want to reinstall OS X, using the Archive and Install option. This option should preserve your user data and leave most apps intact, but again this would be a really, really good time to make that backup just in case.
