It's best not to delete applications that are installed by the Mac OS X Install DVDs. Fixing the underlying problem with the application is usually best.
Most likely you will need an application called Pacifist to extract iCal from the Install DVD that came with your computer.
Your Profile says that you are using SL. Keep in mind that Pacifist is not yet compatible with Lion.
To reinstall iCal:
1. Insert the Snow Leopard Installation disk into the computer.
2. The disk will appear on the desktop.
3. Double click on the "Optional Installs" folder.
4. Double click on the "Optional Installs.mpkg."
5. Click on (Continue) in Introduction.
6. Click on (Continue) in Software License Agreement.
7. Click on (Agree) to agree to the terms of the software license agreement.
8. Click on the disclosure arrow ▶ next to Applications.
9. Select (✓) iCal.
10. Click (Continue).
11. After iCal has been reinstalled, download and install the Mac OS X 10.6.8 Update Combo v1.1 to bring iCal up to the most recent version.
I suspect that it is possible to install individual Lion applications, but I have not yet experimented with that possibility. Ask Ars: Do I have to use the Mac App Store to reinstall Lion? describes how to create a Mac OS X Install ESD. So far I have not used the ESD which I created, but there is a "Packages" folder in the ESD which contains, among many others an iCal.pkg file. Clicking on the package file starts the installation, but I have not gone any further.
Since I have not personally tried a reinstallation from the ESD, I have yet to recommended that procedure.