Computer switches to Safe mode

A friend has a Packard Bell computer running Windows 98. When he starts the computer everything runs fine, then he starts AOL and checks his e-mail. When he opens an e-mail message and after he has been looking at it for a few minutes, the computer suddenly switches to Safe mode and he loses his e-mail. One thing that he mentioned to me is that his power supply fan is not working so he doesn’t leave the machine on for very long at any one time. Could this be related? He has a 25G hard drive with 3G free and 64K RAM.