Ram can go bad. Given the fact your Mac is Intel and it's running Leopard, it could still be under warranty. Any Apple Store or service provider nearby? Otherwise, you could find a local member of the Apple Consultants Network to help you - www.apple.com/consultants
If the folders are shared directly from Outlook, and not from an Exchange Server, you're out of luck. Otherwise, Entourage is your only real bet - the most recent versions give you good access to Exchange Server public folders.
You might try my utility, Mac HelpMate, to give your Mac a thorough cleaning - but you should also check from non-Apple launchdaemons or startupitems (/Library/LaunchDaemons or /Library/StartupItems) as well as any crontab entries that might be triggering a script every five minutes or so.
This is certainly a hard disk or hard disk controller issue. I'd try to boot off of another drive to see if the hard drive will even show up as an option - the installer disk should do the trick, but it it's under warranty, a trip to a service center is probably in the cards for you.
You might need to rebuild your entire Spotlight database. My utility, Mac HelpMate, has a feature to rebuild the Spotlight Index. Just keep in mind it could take some time - perhaps as long as several hours.
Oooh, you might have been bitten by the well-know Mac OS X bug where you can clobber a folder with a file of the same name. However, if you're lucky, you might have accidentally added an extension onto the file that makes the Finder think it's a bundle or some other type of resource. "Get info"...
You'll need to get some sort of usenet reader. Head on over to versiontracker.com and look for one. Then you'll have to choose a usenet server.
There's not a whole lot going on there--it used to be a happening place a dozen years ago.