A good internal Instant Messaging program for my company?

Can anyone advise on a good Instant Messaging program I can use for my
company? Something that would allow:
1) only IM within the company and
2) archiving/monitoring of IM usage.

We were using Yahoo Messenger, but having heard many security concerns and
other reasons decided to stop using it. We are a fairly small company with
about 20 users. Some are in remote offices (connected via Site-to-site VPN
tunnels with the main office). I was hoping there would be something
Microsoft-based that we could use.

I am aware of the Office Communications Server line of products, but that's
a little more then we want to buy at this stage.

Are there any good third-party products out there? Any advice is VERY MUCH
appreciated!