You've bought a computer with an OEM Windows 8 Pro license, and want to run Windows 7 Pro! At the moment, you're allowed to do this, but it's not actually easy.
Ring Microsoft and they will tell you to use retail (though OEM also works) Window 7 Pro media and product key to activate it - unfortunately the automated system may tell you that it's been used too many times and you'll have to activate over the phone via the operator. It's tricky to actually get a product key, too. It was easiest for us to go out and buy one!
Ring your OEM (I rang Dell) and they will tell you that you can just install Windows 7 and it will automatically use the key stored in the BIOS. This is the SLP (System Locked Pre-installation) key. The story is half true - it will work, but only with the Windows 7 media that you can obtain from them, which will typically cost around $55. If you use regular Windows OEM or retail media, it will ask for a normal product key rather than using SLP. The solution feels a bit dodgy, but it works fine. Follow these instructions to create a Windows 7 multi-OEM ISO. Basically, you download the retail ISO, edit the ISO file to add in an OEM folder, then boot from that. No product key, no activation required - hooray!
This works great for a single machine, but what if you have a whole bunch that you want to set up? In that case, you could try using Windows Deployment Service to activate OEM Windows. I haven't yet tried this myself (I opted to ring Microsoft over and over) but it sounds promising.