My pc does not boot when I install 2 ram modules on asus M4A77TD it only boots when I install 1 ram module I tried the other module alone and it worked fine so both of them are fine but they wont work together

Please check the bus speed of the RAM witch one run perfect.After you use the same bus speed RAM in another moudle.

Hi

just go to Asus site from there check you motherboard how much RAM supports

if you have old M/B then it will not support more RAM

also check your OS which windows your using

4 x DIMM, Max. 16 GB, DDR3 1800(O.C.)/1600(O.C.)/1333/1066 ECC,Non-ECC,Un-buffered Memory
Dual Channel memory architecture
*Due to OS limitation, when installing total memory of 4GB capacity or more,
Windows® 32-bit operation system may only recognize less than 3GB.
Install a 64-bit WindowsWindows® OS when you want to install 4GB or more memory on the motherboard.
*Refer to www.asus.com or user manual for Memory QVL (Qualify Vendor List)

Due to OS limitation, when installing total memory of 4GB capacity or more,
Windows® 32-bit operation system may only recognize less than 3GB.
Install a 64-bit Windows OS when you want to install 4GB or more memory on the motherboard.