Overnight errors

I’m running Win 98 on a clone with 1Gig AMD Athos, 512K memory. I get several errors sometime overnight e.g. RNDAL – illegal, WinDLL error and some of my applications same illegal function. Now I’m getting runtime error 204 at 000033A6 during the day. When I close the error window the PC locks up and I have to press the reset to reboot.

Any ideas?