Items in quarantine are in effect neutralized but Sophos should have a quarantine manager and you can delete them using that feature. Sophos is a good product and it's definitely recommended to do on-line scans from time to time in addition to running your resident anti-virus program.
Here's a link to the pdf format manual regarding the Sophos product you have and you will see that section 14 deals with managing items in quarantine.
http://www.sophos.com/sophos/docs/en...xp_76_heng.pdf