Rich,
First things first. More often than not some software was recently installed is causing the problem. You may have added a plug-in, updated your browser to a later version or added some software that may seem to be totally unrelated to the problem at hand.
Win98 has a recovery feature that will allow you to roll-back to a previous (5 or so) login. However, you may be too late for this to help.
When you receive an illegal function call, look at the details of the error if you can. It may help you to determine what software actually blew up.
On the other hand, it is Windows and you could be in what is called DLL (for Dynamic Link Library) H E double toothpicks.
You might try uninstalling any plug-ins or software installed recently and reinstalling the software that was working. This might clear up the problem. If not, you may be looking at a reinstall of the operating system.
MAKE SURE YOU HAVE BACK-UPS OF ALL YOUR DATA!!!!!
This includes your e-mail files, bookmarks (or Favorites), and a copy of your autoexec file that has been modified to boot from a floppy and allow you to get to your CD. Without this, your computer is nothing more than a paper weight.
Not trying to scare you, but I do this and other computer related stuff for a living and I speak from experience.
Good luck.