A peek at the elegant "wonderware" selection of software on this site will give you an idea about how secure instant messengers really are. Is postal mail beginning to look better to you yet?
The program allows up to a thousand different ICQ or AIM windows to be open at one time (i.e. several different UIN's could be running at once). So, be careful. Who you thought were two different people could be one person who is just fooling you!
ICQ Bulk Pager
If you dislike spam, you will probably not want unsolicited instant messages, either. This program allows spammers to send messages to millions of ICQ users all at once.
Secure Shuttle Transport (SST)
If you really want to use instant messaging, you may wish to consider this program. SST allows audio (voice) communication, text chat, conferencing, instant messaging, file transfers, and email attachments to be automatically encrypted with RSA encryption technology before transmission over the Net.
Top Secret Messenger (TSM)
TSM is based on the Elliptic Curve Cryptography (ECC) algorithm. Please note that the limited trial version supports only very weak 8-bit encryption.
PGP-ICQ is a free plug-in application for ICQ that allows you to encrypt your ICQ messages using Pretty Good Privacy (PGP).
A few articles about the perils of instant messaging are mentioned above - this is just a small sampling. A few more secure alternatives
are also suggested. As with all software, etc., try any of it listed here at your own peril, and it is always wise to make a backup of your system before installing anything new.