I have a Dell Inspiron 5100 and upgraded RAM to 1GB and hard drive to 80 GB. I have to use an auxilary fan or it will overheat and shut down. I've been told it's probably the HD. Should I switch back to the OEM 60 GB or what? What if I went to 100-120 GB? I have additional storage, so I can offload pics and music to another HD if necessary.

I'm not sure if I understand your issue but I doubt that changing your hard drive or upgrading your ram would cause it to overheat. Make sure your system fan is spinning. You may also want to buy a can of compressed air and use it on all the fans and openings in the machine. Make sure you hold it upright so no liquid escapes into the computer. The goal would be to remove all dust and dirt from the machine. If you are still having problems you can email me at shane@driventechnology.com. Hope that helps!