Do the services exist? If so I'd try a manual deletion of the services. I'd also do a reboot as sometimes services are marked for deletion but do not go until the reboot is completed, Finally see which services are installed and then from a command prompt do a sc delete servicename. Reboot and that should clear down existing services so you can complete the install.
The most common cause for this one is knowledge. There are two options for the knowledge builder service. You can have either the Active Knowledge service or the Core Knowledge service. But if you didn't know any better and were trying to get a full install then you might check to have both of them installed. It would install the first one, and when it goes to install the second one (which is just a slightly different flavor than the other one) then it would see that you already have the service installed and error out.
Check to make sure you only have two of the three knowledge options checked to install, the Knowledge Management Engine and one of the two Knowledge Builder services.
None of the suggestions worked, was easier to blow the server away and get a new VM cloned.
I had a similar problem. After installing 7.3, without using 7.2.6 uninstall first.
I tried to uninstall 7.3 two times in a row, got different error each time. Then I used the command line tool SC DELETE to manually remove services.
Then I was able to install 7.3 correctly.
I learned the hard way to always uninstall the old version before installing the new regardless of what the manual says.