Threat detected in quarantine file -- significance and response?

I loaded and ran Sophos earlier today on a new Mac that I set up using Migration Assistant to bring my files from an older computer.  The older computer had its own antivirus software (Norton for Mac, Version 2.1.1 (5)), and that application was ported over in the migration.

Sophos found one threat, Exp/Datbi-A, in the file /Library/Application Support/Symantec/AntiVirus/QuarantineFile.qtn.  Does that mean the threat is somehow masquerading as a quarantine file, or is this an older threat quarantined by Norton and migrated over still in quarantine?  in either case, what's the appropriate response?  This particular threat may not be an immediate problem, since the threat description says it's an XML exploit attacking IE7 on Windows and I'm running Firefox on Snow Leopard (not currently set up for dual boot), but I don't want to take anything for granted.

Thanks in advance,

David