When I start my Cisco router it shows me the attached error. What is the problem with the NVRAM of the router and how do I fix it? Thanks.
The problem is exactly what is stated in the message you see: the nvram (non volatile ram) of your router is corrupted.
You need to change it.
It seems it is something quite easy to do, provide you get a replacement flash SIMM.
Cisco provide the procedure here:
Note: in some routers the NVRAM is different from Flash. It seems in the 2600 the NVRAM is actually on the FLASH but I'm not 100% confident and don't have time right now to check it.