The data you entered into iCal, iPhoto etc. is part of your User account and not the operating system. So re-installing the OS should be ok as long as you keep the user accounts.
I would look into checking your fonts first. I believe iCal uses Helvetica. You may find some useful info in this post: Default iCal icon font?
Thank you, yes there's definitely something wrong with the Helvetica fonts (though at didn't even touch the, when I installed the new ones). Is there any way to repair the font system without reinstalling the entire OS? I've tried validating from within Fontbook & found a number of serious errors which I resolved but the iCal/iPhoto problem persists.
I managed to fix it. I simply rebooted in Safe Mode as I figured this would rebuild the font cache. Which it did, or at least it did something as both iCal and iPhoto are now working fine again.
Thanks for your help!
PS Anyone else with this problem? Here's how to reboot in Safe Mode: http://support.apple.com/kb/HT1455