There may be a variety of reasons why Windows won't start after adding new memory.  Firstly, you have to make sure that the BIOS recognizes the new memory.  If BIOS is okay and Windows is still not cooperating, try to install modules individually to isolate any defective module that may have been causing the hang system, that is, if you have more than one module installed.  If all memory modules work individually but not together, then you may need to do some serious diagnostics to identify the problem.  Microsoft Online Crash Analysis offers a comprehensive diagnostics program in order to correct Windows errors related to memory.