| Disk utility is good, a first start, booted off the OS CD to use it.
Then there's TechTool Pro and DiskWarrior
For viruses, Virex. You can search the forums for opinions for virus software. There are no viruses for OS X, but you can pass them on through email/attachments.
You have a built in Firewall. Look in the Sharing PreferencePane.
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