It sounds like the system still has the program installed. You need to totally uninstall the program first to make sure the installation program can work properly. Something may have deleted the program, but not removed the information from the registry. If this happens, the program thinks the install has already completed. Which is why it sounds like it is trying to uninstall the program first. You may have to get some professional help here or do something you may not like to do. Someone that works with software / registry issues can usually find what they need to remove (some programs can do this too - Norton pops to mind). Be aware that if you remove the wrong thing, you can cause hangups, crashes, non-rebootable issues. Something else you can try is to totally reformat your system and re-install everything (you should make backups of important stuff before trying this method and make sure you have an installation CD with installation diskette - if needed). In order to get a clean installation, it sounds like you have to remove whatever the program is finding.

What is the program?

Still having problems?

Like Luvian said, it would help to know which app is causing you grief. If it's freeware or shareware, then that could be the problem.
I'm taking a guess that your OS is 98, ME or 2000. If that's the case then here's how to check your registry to see if the default install path is messed up.
CAUTION: You are entering your registry. This is serious stuff. If you are the least bit unsure of what you are doing, DON'T DO IT! It's very easy to completely bork your system in here. That said:

1. Open Regedit and go to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\Curr entVersion.

2. You need to change two values to reflect where you want the default installation directory to be: - ProgramFilesDirectory - ProgramFilesPath

3. Double-click those values and edit the value data as appropriate.

The default pathway should be c:\Program Files