Lisa, the computer is seized as evidence and the time/date establishes who was home at the time? Is the computer password protected? There are lots of ways that forensic IT can show who was on the keyboard at the time.
Even gossip can bring you unstuck in defamation matters- spead a rumour about someones sex life, professional competance or moral standing and you could be gone big time.
Damages awarded against you could be as high as $250k with costs of $100k+ for each side. Do you have $500k to throw away by saying that some body is, say, soliciting prostitutes on overnights?
If you say some thing, the onus is on the utterer to prove truth of the utterance. There is no burden on the defamed to prove their innocence.