Windows 98

Boot computer and when you see the Illegal operation box click on details and see what is noted.
It may be a corrupt software file or hardware problem that is causing this,but I would first run the anti-virus to check system
Then run scandisk to check the drive for any errors
Try getting more info on the problem and then determine if you will need to download updated .dll or hardware drivers for your system

This problem could also come from improperly removed software and a registry error.

Go to the RUN menu, and run MSConfig.
See if any of the programs on the startup tab are programs that you no longer need or have, and uncheck them.

It might be a good idea to run a registry cleaner such as RegClean from Microsoft.

I would go to details and find out what program or hardware driver could be causing it. Then take it out of startup by going to start menu, programs, then startup and seeing if is in there, or by going into msconfig. Just go to start menu, run and type in msconfig and then go to the startup tab.
Also to make sure it isn't a hardware problem also I would right click on the "my computer: icon, click on properties then go to device manager and see if you have any splats (yellow exclamation marks) on any of your devices in there. If so you need to resolve them by getting the proper driver or addressing if it could be a resource issue.

This is a VGA driver problem Update VGA Drivers with latest or Win95 Driver.
Most Tech don't agree but it work's for me.