On my emachine running xp home, a few minutes after start up it cpu fan shuts down and the monitor looses signal. If I run in system bios, the cpu fan does not shut down. Any idea what may cause this?

Hello, and Welcome to D-A-L.

There's not a lot of CPU use while in the bios. It sounds as though you have an overheating problem. Check the CPU fan and heatsink for dust buildup. Also ensure the heatsink/fan assembly is tight against the CPU.

in the bios you can change the shutdown temperature so that your pc stays on longer while it is hot or you could just by another fan for your pc

You can also damage or possibly kill your CPU by altering these "safeguards".