Try the steps mentioned under Method 3 (Option 2) in the article below and then check if you’re able to install Office.
If the issue still persists, then download Office Professional 2007 from the link below and then try to install it using the product key that you have.
The steps stated in the Microsoft support site are useless. My software too (Office 2010 Professional Plus DVD) has the osetup.dll missing error. My problem is that I am unable to copy the 'troubled' software to the harddisk in order for me to move ahead and do the other steps to replace the errant dll.
I am unable to copy contents from the DVD to the harddisk, because I get the error message "cannot copy.... cyclic redundancy check." There is no support available anywhere I checked, for this situation.
I purchased Office 2010 original DVD 5 months ago. I was able to install the DVD then. Due to problems related to add-ins which started crashing MS Word and MS Excel recently, Microsoft support asked me to uninstall the software and reinstall. I uninstalled successfully. Now, I am unable to re-install. Now, even MS support is unable to help. They have (over phone) spent over 6 hours of my time in the last 3 days. They are struggling for solutions. The retailer who sold the DVD is asking me to contact Microsoft for further instructions. I am stuck!
I know people who use pirated software without any such problems. I have always used legal software. Is this punishment for purchasing original software?
Additionally, I had to change the permissions on the folder I copied the files to. It was Administrators group owned, and my account is a member of this group, but I was unable to run the setup.exe file until I reset everything to be owned by my personal local account. Hope it helps someone.