|"they put thermal grease on the cpu"|
That may or may not be the problem. Thermal grease (aka paste) must be installed properly & the method of application varies from CPU type to CPU type. Some people make the mistake of laying it on like they're frosting a cake...that's 100% wrong. All you need is a tiny dab about the rice of a grain of rice (middle dot method) or a thin line of paste running down (or across) the middle of the CPU (vertical or horizontal line method). And the paste should NOT be spread around.
Assuming you have the Gateway GT5240E, you probably have an Athlon 64 single core CPU. If that's the case, the paste should be applied using the middle dot method:
Middle Dot Method