Windows 7 - Odd win 7 installation errors?

Here's the story. I took a client's HDD, which could not install Windows 7 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 Windows 7 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 Windows 7 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 Windows 7 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 Windows 7 in the dropdown menu, boot on one-time BIOS Boot Menu key under USB, Removable or HD's.