In order to troubleshoot the issue, kindly refer the following
Run MSI Cleanup utility
Download the MSI Installer Cleanup utility and install on the system in question. It can be obtained from following URL:
http://www.microsoft.com/downloads/details.aspx?FamilyID=5fed58f0-8b41-4928-8939-328452843bce&DisplayLang=en
After the application is installed, run it from Start/Programs/Windows Install Clean Up
Reboot the computer and then try uninstalling Backup Exec software.
We hope this helps.