I've just installed a fresh elementary os freya to dual boot with my windows 10 but when I startup my computer it goes straight into elementary os and skipped the grub bootloader screen
Can you try to hold shift key when booting up?
If there is no Windows, boot elOS, open terminal and execute
sudo update-grub and reboot
Then you should see it
And read this
Please try keep pressing up/down keys when your system boots, if there is no effect try this:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair && boot-repair
And then click on recommended repair
then follow the steps according to the instruction.
And then if you don't get the option of both OS, there should be issue of UFI boot.
Reference link: https://help.ubuntu.com/community/Boot-Repair