Depends on how to define "Instant Messaging" and to what lengths you want to go. I can go on Google Talk through my browser on HTTPS. So unless you're blocking gmail or somehow convincing your employees to accept you as a MITM on their email - you can't monitor that.

If you want to monitor AIM, MSN, pr ICQ in a simple configuration - yes you can, and there are products ranging from simple to enterprise for doing so. Ask your Networking Vendor or google a bit

There are numerous ways to monitoring instant messaging communications, but your question is rather vague.

Is this monitoring within an organization or on your home systems?

It may be possible to setup systems that may record/relay communications across the default ports of various IM applications (Yahoo, MSN, AOL, etc) within an organization or at home.

If you can expand on your question, we can provide a better response.