Well, there really is no single centralized AV. You can put AV protection on the mail gateway, the proxy server (for browsing), the mail server, the file server, the clients, etc. Trend tends to make a good suite of products as does Symantec.
Some people recommend "defense in depth" - Block everything you can at every layer you can. Example: Have an AV scanner at your SMTP relay, on your Exchange server (If it's a different box than the SMTP Relay, which is very common) and on your client. If possible, pick at least two different AV products - If one isn't updated or doesn't catch a particular variant, others will.
That being said, there is NO substitute to a well-managed and well-deployed workstation AV strategy - It's definitely the most important. The others can be too easily bypassed (i.e. HTTPS content can't be scanned at the web gateway, password-protected zip files can't be scanned by the mail relay, etc.)
- G