If they will not impact my mac, and are for windows only, how did they end up on my computer?
The most common way to acquire Windows viruses on a Macintosh is as file attachments to incoming e-mail. They might simply have come attached to malicious spam, or they may have been sent out by a Windows virus that propagates itself by infecting a Windows computer and mailing itself out from there. (It which case the viruses uses the address book on the infected Windows computer.) So they may have even have shown up apparently having been sent by a friend of yours who uses Windows.
Look in the Attachments folder for your e-mail program and see if the Trojans Sophos located on your Mac are residing there. If you are using Mail as your e-mail program, you can find your Attachments folder at:
[hard drive icon]/Users/[your user name]/Library/Mail Downloads/
If you can't recognize the Trojan Horse files by name (which is likely), it might help to use a different anti-virus program. This one is free:
ClamXav (free)
ClamXav
You can use ClamXav to scan just your Mail Downloads folder, or your entire user folder, or your entire hard drive; whatever you prefer. ClamXav will allow you to delete a Trojan Horse with just a click.