When it comes to PCs, it's nearly impossible to ensure anything.
I'm not exactly sure why you'd want to replace 8.1. Overall it's a better OS in terms of data processing and efficiency, and going forward, Windows 8 will be the standard until Windows 9, so it's better just to stick with 8. With all of MS's changes to mitigate the metro stuff, it's not even that different from 7.
That being said, there are still plenty of legitimate reasons to downgrade (like legacy software), so if you need to, just hold your breath and hope.
If it's a pre-built machine (OPs case), you can navigate to your machine's driver page and see if they have any drivers. I also tried to downgrade my HP Envy-dv7 (I got it when win 8 was brand new and still aggravating), and my rig was completely decimated. 800x600 resolution, no networking... Nothing worked, and HP support just told me, "We don't support installing new OS's, and we're not releasing win 7 drivers" (paraphrased).
If it's a custom build (possible reader's case), then you're probably in luck. Most parts manufacturers in this author's experience have support for 7,8, and even sometimes Linux. Just navigate to the drivers' pages of your various parts and see if they have 7 support.