HELP! Need reliable, affordable Enterprise IM Solution.

Hello-

I have been challenged by my boss to revamp our IM needs. Currently our 70 person company relies on net-driven solutions like AIM or MSN Messenger.

We have picked up some healthcare clients who are requiring a more secure means of contact.

Can anyone recommend a low-cost, easy-to-administer, and most importantly, HIPAA compliant instant messaging solution?

Thanks.