Sudden Freeze

My computer suddenly freezes without any reason. I cant even move the mouse and the only choice left is to reset it.
I have the latest drivers, Bios, windows Patches, etc. What could it be?
My system is running on:
Windows 98 SE
Pentium III 733(133 Mhz Bus)
Nvidia GeForce DDR video card.
192 MB RAM (133 Mhz)
Aopen AX63pro Motherboard.

Hope you can help me.