That sort of thing is not possible due to how the anti-virus system has to work. This is summarized some some extent, but the concepts are there.
For on-access scanning, which is by far the most common type of scanning activity an AV package does:
- A program attempts to open a file. It does this by using Windows API calls.
- The Anti-Virus program has hooks into the API calls and is executed instead.
- The AV program scans the file for viruses.
- If the Av program finds none, it allows the program to open the file.
Because this "gets in the way" of running a system on naked disks, AV programs necessarily introduce some lag in accessing files. Users notice this. Because of this, the scanning engine has to be as close to the user as it possibly can be, and that means locally. If step 3 there involved copying the file over the network to the Scanning Server before it is released, the lag would be very, very, significant.
Also, modern AV packages also do things like scanning running memory for viruses, which is something a remote server simply couldn't do without a lot more Windows API hooks than currently exist. At minimum an agent would have to be installed, and protection still wouldn't be that good.