Since you just installed that hard drive,. it may have taken over priority from the other drive
Not sure about this one, but it;s worth a try:
1. Insert the Windows 7 DVD
2. Restart your computer
3. When asked if you want to boot from your DVD drive, do so.
4. Choose your language, click Next.
5. Click 'Repair your computer'
6. Select operating system you want repair.
Select the Command Prompt option from the recovery menu
At the command prompt, change the drive to your optical drive
type: x:\setup.exe
My Google search revealed this a s a common problem with the RC,., but not the purchased disks.
Have you tried the 64-bit version?
It could be a corrupt disk also.
Could you please fill in your computer specs in the UserCP link at the top left hand side of the page.
This freeware program will give you all the information you will need:
SIV (System Information Viewer) 4.03