Right click on start and Goto Device manager and find CPU under heading processor. Now you should right click on each and every processor and click on uninstall.
Next you have to reboot the computer and now the newly installed processor will be recognized.
Even after recognizing if the computer is still slow, then check for the speed of RAM, the amount free space the system has then find the suitable replacement. The speed not only depend on the Processor but also on the RAM.