Here's my tidbit of advice on how to avoid viruses in the future for what it's worth.
If Sophos antivirus is causing performance and web issues, then I strongly suggest ditching it and complelely uninstall the software per the directions on the Sophos website. OS X doesn't really need any antivirus software, but there are many here who'll argue the point. Running any commercial Antvirus software on OS X is going to negatively impact computer performance in one way or another.
