After checking the information you provided, I might guess your laptop computer is Asus K61IC. You could reference this page, https://www.asus.com/Notebooks/K61IC/specifications, to inquire more information about the supported processor list. Intel Core 2 Duo T7500 (which also has a 4MB L2 Cache and supports Intel VT-x feature) has similar features with Intel Core 2 Duo T5900, so the firmware of your laptop mistake your processor with the former. Core 2 Duo T5900 is an OEM processor, so I could not find more information about it. NVidia MCP79D is a charming chipset, especially for its integrated GPU and support of the boarder range of processors and memory modules. But changing the processor within laptop is not an easy job, wrong operations might also damage your laptop permanently, so be careful and everything is at your own risk.
Thank you for contacting the Intel® Communities.
Regarding the upgrade room for laptops; This is very limited and it depends on the OEM, they are going to be providing with a list of processors that can be used in their device if available.
I was checking the Manual for the Asus K61IC and I was not able to find any compatible processors noted there; This could mean either there are non or the information is present in another site.
My recommendation for this would be to indeed contact the manufacturer of the device and request support on this, they are the ones that designed and integraded the device; therefore, they have the accurate information about whether or not the processor can be upgraded in this unit.