Tell your friend to stay away from your computer!
I am guessing this is a Vista and FireFox issue? Make sure you are logged on as administrator. I would be tempted to reinistall Firfox and the addons again. Alternatively, uninstall an addon and reinstall the addon one at a time and see if that has an effect.
After reinstall, go to adobe and try installing Flash again, does it install in IE ok? Ensure your Firewall is not interrupting the process.
Other than that. . . try another forum as this is specifically for HCL software.