Hi Welcome to FixYa!
1)Are you the only one who is using the computer? If not, be sure that you are or you have Administrator rights to install anything on the computer. Because if your account is only a standard user type, you dont have privileges of installing any software or device on the computer.
2)If you are the Administrator of the computer, try to right click on the installation file and select Run as Administrator and see if this will work
3)Check also your security softwares installed on the computer. (e.g Antivirus Softwares) If you have them installed, try to disable them first and run the installation file for your webcam
4)If it still doesnt work, do a clean boot on the computer. You could do this by going to the System Configuration Utility. (type msconfig on the searchbox to access this utility)
-you should be seeing something like this:
once you have a window like this, go to the startup tab>click on disable all>click on apply, then go to the Services tab>put a check on the Hide all Microsoft Srvices option>then click on disable all>then click OK
-it will now prompt you to restart the computer
-once you get back to Windows, try now to reinstall the software for your webcam
5)if all else fails, your last option is reinstalling Vista on your computer.
Hope this would be a big help!