Laurie, right click my computer > properties > advanched > performance > settings
Set it to optimal perfermance. It will not solve the problem but it will make your system run more decent.
If you really want a IM, give Trillian a try. It supports AIM, ICQ, MSN, Yahoo Messenger, and IRC
It has been a while since I tried it, but back then it was lighter on resources then MSN, YM and others.
Have you checked if it makes a difference if you have the IM provider in Avast! on or off?
Vervain, your problem has nothing to do with this thread, please start a new one.