Sometimes not being able to download or install a program is due to interference from other programs such as antivirus and antispyware. Whenever downloading/installing a program, be sure to do a “clean” program install, i.e. do the downloads/installs with all non-Microsoft services disabled. Type in msconfig in the Run box and press Enter to display System Configuration. Select the Services tab, check the box for Hide All Microsoft Services, then click on the button for Disable All, then OK. Reboot. Do your download and install then go back to System Configuration, Services tab, select Enable All, then OK. Reboot to get all services running again and you’ve done a clean program install. If this doesn’t work, look into install/uninstall debris problems.
Install/uninstall problems are common with Vista and lead to the accumulation of debris from broken installs and incomplete uninstalls which can block or corrupt normal download and install processes. Before you go through the steps of Repair Vista Options, take a look at this tutorial about install/uninstall/download problems (link provided below). After you get install/uninstall debris cleaned from your system, if you’re still having difficulties then go through Repair Vista Options starting with step #1 and so on until your computer is working right.
In the install/uninstall tutorial I would advise using Ccleaner to clean up and shred unnecessary files and then Method Two using Smarty Uninstaller to remove install/uninstall debris. If this isn’t enough to get downloading and installing working again, before reinstalling Vista, consider using a registry cleaner (links provided below) to deep clean misinformation debris from the registry. The Wise program is free and comes highly recommended. I like Registry Booster (not free) myself and have found it very useful for this kind of problem (set it so that it automatically makes a backup of the registry before it makes any changes; Registry Booster will also defragment your registry).