[Simple]

You could also mention how much RAM (if you bought the computer with 32 bit, then it is likely to be 4GB) and Disk space you have.
But it is most likely the Disk and I would not advise you to change the type, since you are not familiar with BIOS/UEFI.

However, while I am not familiar with your laptop, if you wish to change the controller type:
1- Create a small partition (10 mb) on the Disk Drive of the type wanted, and/or change it through Disk Management.
2- Format it and let the computer recognise it (this is for Windows to download the appropriate drivers).
3- Now you can change the Disk controller type (in BIOS) and reboot from you original Disk.
4- Remove the small partition you created.

If you don't do the above steps, you won't be able to boot. There are other steps between the above ones, but they are the main points.

Having said that, I would install Windows 10 64 bit, then install VirtualBox (fast and free) or VMWare player (free and good compatibility) and you will have a lot of advantages using 32 bit Windows XP as it requires few resources.