Recently, every time I start up Windows 98, I get to the start up screen but the computer just freezes (I can move the mouse pointer, but can't right-click or click on any of my desktop icons or access the Start menu). The PC fan is whirring away very loudly as well. When I press CTRL ALT DEL, it shows that explorer is running, and if I try to close it, I get a message saying "Explorer - this program is not responding...".
I've tried rebooting in Safe Mode, and the same thing happens - I can't get any further than the start up screen.
Does anyone know what's causing this/how to fix it?

Any help would be much appreciated!
Thanks,
Gez