The windows CD's are themselves bootable. A quick search of microsoft's website for "Unattended Install" would yield information on how to automate the installs. I'll warn you though, by the time you read through the info and lab it out and have a working unattended install, you could have done a manual install of all 20 PC's by yourself.
Your best and smartest solution is to connect the PC's to the network, create an image and do a network boot via a PXE NIC or network bootdisk and run the install from there. With an image, you can have all software needed installed within the image (including service packs and hotfixes etc) and once started, the process is automatic (ie: you don't have to answer prompts and be sitting at the console). I've done many many installs this way and prefer it.
All in all, any way you slice it, you have to spend time. Either doing the installs manually - including updating the OS and then installing needed software - one PC at a time, or, you can spend the time up front to create an image and then deploy it en mass via a network share.
Having done many OS installs both ways, I prefer using an image whenever possible. Once you've spent the time creating a process for doing this, in the future when new hardware is deployed, you only have to spend time creating the image for that hardware, the rest has already been done (ie; network share, network bootdisk etc). In the long run, this means less of your (or someone elses) time spent doing installs manually one PC at a time.
