Android Reboot to recovery if volume buttons are not working

Step 1. Power Off your android phone
Step 2. Open FastBoot (for eg. in HTC Desire press Back and Power Button )
Step 3. Open CMD on your windows machine, move to sdk/platform-tools. where adb and fastboot exe are  
            present
Step 4. Run the following command
                   
                                fastboot oem gencheckpt

Done ! your phone will reboot to recovery

Source :XDA