Odd win 7 installation errors?

Here's the story. I took a client's HDD, which could not install Win7 due to lack of CD drive, and tried to install Win 7 using my home PC. At first, I tried to install but the installation disk said it couldn't locate the correct directory. I then unplugged my two personal harddrives whilst keeping the blank one in while the computer was still on. It wasn't until I restarted my PC that I was able to install. But at the set up your PC stage (name your PC, set password, etc) I turned the computer off, hoping to continue the process on the actual PC that I was going to keep it in so my client could customize it. After plugging my two personal HDD's back in, which were working fine just 15 minutes prior to this, I discovered that neither of them could boot. They just hung in the Win7 loading page. On safe mode, they would get stuck on classpnp.sys. Now, my computer just says that disk cannot be read, and that I should press CTRL+ALT+DEL to restart.

What should I do?

I know that:
My RAM is okay.
My HDD's data is still intact.
My HDD's are NOT broken.
CPU is not overheating.

If you install Win7 in other hardware you must SysPrep the HD or it's image before moving it back to the target computer, so that it starts up like new with no driver conflicts, SID/HID or Activation. Only rarely will it start up on its own, usually hanging at the Startup animation when drivers load.

You would not need a CD drive in the target computer as Win7 is on a DVD which are easy enough to borrow or cheap enough to buy. Or you can write the ISO to a flash stick using Universal USB Installer with Win7 in the dropdown menu, boot on one-time BIOS Boot Menu key under USB, Removable or HD's.