How are you trying to install, and what? Trough Mandriva's package manager, by double-clicking on an rpm file, or something else? If you can, post the error message as it's written, it might help also. You might get more information by running the install app from command line (because errors are typically sent to stderr, which in turn usually ends up into the terminal you're using, but you wouldn't see that if you didn't launch the program from a terminal); if it was the package manager you used, try to launch it from a terminal and then re-try installing, and see the last messages written to the terminal when the problem occurs. Or if it's a single .rpm file installation, open a terminal and either become root ('su -') or use sudo ('sudo rpm -U package.rpm') and run the rpm command:
rpm -U package_name.rpm
Again the last lines should tell what's going on..
If it's a big problem and you can't find a solution, a reinstallation of the system typically helps, but that's quite a big step and I wouldn't tell anybody to go that way unless it's absolutely a must
(for example, if there was only hours to get the thing working no matter what, and no solution in sight, but backups already taken).
It's easier to help if we know as much as possible about the problem. If you can, copy and post here the commands and possible error messages you faced, or if it's a graphical program that's troublematic, take a screenshot/series of screenshots, upload them to some image service (like imageshack or something like that - a service that you can freely upload your images to for a period of time, after which they usually get deleted) or your own webspace and post the URL to them here. An image usually tells more than a thousand words, or so they say..