Once in a blue moon without the intrusion of worms virii or trojans, data will be lost from the hard drive due to corruption or 'misplaced' by defragmenting.
(Very rare)
Not sure what could have happened in your case as an entire address book file is kinda hard for that to happen to.
I could see info within it being corrupted.
If you have norton utilities, open Norton Unerase and see if it got dumped for some odd reason first.
After that, update virus definitions and scan repeatedly.
Sometimes there is a virus and the scan misses it on the first pass.
It is possible a worm or trojan did that, yes.
That actually is the most likely reason.
But check the unerase files (if an option) and scan again after updating virus definitions.
Yesterday I got dusted with a neat little deal that remapped the "Alt + Left arrow" hotkey to my "O" key.
No data corruption at the moment, but it took awhile to clean the machine.