Take a look at This Discussion
over on the Apple Support communities. (edit to add and This one too
The MacKeeper issue has been discussed before as a malware threat that is delivered as an offer to prevent malware.....also seen under the names, MacShield, MacDefender and MacProtector.
The malware topic for the Macintosh remains predominantly on the side of running without additional software beyond the internal protections provided by having an up-to-date Operating System with all security updates current.....for now. As the Macintosh OS becomes more of a specific target by the malware producers we will see the potential for increased security (anti-malware software), but for now, with less than 30 actual threats identified, I haven't found it worthwhile to put up with the additional intrusion that comes with running anti-malware software on my personal machines.
While you are still on 10.5.8, you may have some vectors for intrusion that have been closed in 10.6 and 10.7, but for now you would most likely be asking for more challenges by installing a virus checker than you would be facing otherwise.
If you are currently running Sophos free and it doesn't intrude on your normal workflow it won't hurt to keep running it (but it probably won't provide the help you are expecting from it). Just remember that since anti-malware software needs to follow a list of definitions to recognize the threat, and currently there is no viral threat in the wild (only trojans which require your permission to be installed), then there is currently no protection for the as-yet-unidentified threat lurking on the far distant horizon.