Hi,
If i understand this correctly, You want to protect the fedora usng the AV which is installed on windows vista.
The two OS are installed on different file system.
1. Windows Vista will be installed on NTFS file system.
2. Fedora in that case requires unallocated space which is not been identified by NTFS file system.
Due to which you will not be able to use AV installed on windows vista to protect Fedora.
The threats which out there in world are specific to file system so in that case you need different antivirus progrma which will be specific to OS.
To protect fedora you can try using symantec antivirus for linux.