Software not installing/working

I have an HP 6300 Pro Desktop computer that I just loaded with Windows 7. Installed software on it (QuickBooks and IBM iAccess). Software worked great in my set up lab. Brought the computer to the user and software worked great. Head back to my office and they get an error on the QuickBooks software that it will not open the file. I have researched QuickBooks support up and down and tried everything to no avail. I removed all software (QuickBooks and IBM iAccess) and tried to just install QuickBooks. Still getting error message. It is just weird because they had an older PC that the same thing was happening which is why I gave them the new PC. The software works alone and with the IBM iAccess on 4 other PCs. It is not a license issue as I am not even able to open a sample file in QuickBooks. Anyone please help as I am out of ideas to get this software working. Why would it work and then just stop?