Sophos have recently announced a free version of their Antivirus software for Mac. The software has low system requirements and will find and quarantine Mac and Windows viruses, trojans and worms.
One word of caution comes from a Mac user who lost his entire Time Machine backups while using the new software. Sophos have been quick to mention that their software has been used on Macs for many years, and by hundreds of thousands of new free users without problems.
What this really shows is that if you delete files from you mac then your Time Machine volume is no longer a backup but the only live copy of the files. Either make multiple backups or leave the files on the original volume after backing them up. Also be aware when installing new software that you should have backup copies of your data in case it all goes wrong.
It appears that this guy was having some problems with Time Machine before any of this happened.