Firing my boss

Have the lock changed on the Server Room and Backup, Backup, Backup. Why would they expect a problem with your boss? Is he going Postal?
Desktop Copies of Critical Apps come in handy also.

You didn't mention what kind of network you are protecting, that would be helpful to others answering your question.

Do you have remote access for this person?

Do you run IDS (Intruder Detection Software), if not check out snort.org (excellent freeware).

In addition to the useful ideas in the earlier posts:

Make a backup. At least if your boss does manage to get back in you can restore things to a recent working state.

Make sure that all administrators change their passwords too. One of themmight have been compromised. Ditto for your routers, firewalls and other managed network appliances. Rename the administrator account. Check for unknown accounts. Check membership in privileged groups, you can sneak an ordinary looking user intoadministrators, backup...

You might want to create a checklist of things to do *while you boss is in the office getting fired*. This is the time to change passwords, restrict accounts, etc. Depending on the situation, if the boss gets wind of abig security sweep and he's not part of it, he might get suspicious. If he is part of it he'll be aware of the new passwords and such (bad idea).

If you're really paranoid, set working hours on accounts and set accounts to logout when their hours expire. It's amazing what you can usually do with a normal account and no supervision.

Turn on full auditing. You will probably generate lots of logs. If something does happen you can turn the data over to your legal staff. If nothing happens, after a while you can set auditing to normal levels.

Turn on and check your firewall logs. Check the firewall rules to make sure they still implement your security policy. Some firewalls have time-based rules. Consider severely restricting access except during working hours.

If your boss has a sympathetic administrator it is possible that they will work together to breach your system. This is why auditing, review and time-restricted access are necessary. You have to trust someone but the principal of "least privilege" should always be applied.

Good Luck

Here's the steps I usually follow for any terminating employee:
1. Review level of employee's access and permissions for all shared resources. If the employee works in the IT realm, an in-depth review of all enterprise resources the employee had access to is necessary.
2. Block access to all shared resources other than user's home folder, email, and printers (disabling accounts and blocking access before the fact will definitely give the employee an inkling that something is up). If possible, monitor employees access to email and the Internet. Taking this intermediate step (in lieu of immediately disabling accounts) may not be necessary if the employee to be terminated already knows what's coming. You may want to get guidance from management or the HR department before just pulling the user's "plug."
3. Make a copy/backup of all data in the user's home folder, e-mailbox, and any business-related data residing on the user's computer.
4. Changing the administrator passwordmay or may not be necessary. This will be determined by the terminating employee's job. If his job is anywhere in the IT realm, I assume he had access to the administrator password and change it.
5. As Answer 1 suggests, physical security issues must also be addressed.
6. On the actual day of termination, I show up early and disable the user's account(s), both domain and local computer if applicable.

Hope this helps.

In addition to all these other suggestions, I check the users PC itself to see if their are any usernames that are not company standard. If there is one there is a chance that same username exists on the network somewhere with special privelidges.
Another good thing we have done in the past is to make a image of the users hard drive. One of my previous employers would do this in case a unhappy employee decided to try to blackmail or sue the company. Anything on media can be admitted as evidence and if your boss isn't the best at covering his tracks those mp3's, jpg's, and IE history logs really help out.

P.S. The network consist of Windows 2000 server, NT server and Citrix server for dialup access. I currently am looking for Intruder detection software. Any thoughts on IDS software??