Anti Virus On Access Scanning

Hi,

I hope someone can give me some advice as to which is the best way to
configure a real time scan, should it be to scan files on read or on
write.

The reason why I ask this is that every AV scanner I have used seems
to have the default configuration set to scan files on read.

we do get viruses from time to time and usually they are cleaned up by
our AV scanner (set to scan on read) however sometimes a virus will be
detected but cannot be deleted (I am assuming the file is locked some
how).

If the realtime scan is set to scan on write should it not prevent a
detected virus being writen to disk in the first place?

or am I misunderstanding the way these scanners work?

Are there any disadvantages to setting the option to scan on write.

Thanks

James