Installation help

Hi. I'm having problems with the set-up for the game and I REALLY wanna play. I'm fairly certain my system meets the requirements and I know I have plenty of space for it. Here's my problem:

When I try to install it, I get to the Screen with the loading bar, after selecting which folders I want the game to be in, and I get one of two errors, depending on whether I changed the default folders or not.

When I change the destination folder to c:My DocumentsDownloads, and the program folder to Atari (I previously attempted to install the first RTC game, but couldn't because of a similar problem, that's why I have that folder), I get this error near the end of the install:

Component transfer error
Component: New Auto RTPatch Client Installer 6.50 US
Error: Access is denied

When I don't change the folders from there defaults (destination folder: C:Program FilesAtariRoller Coaster Tycoon 3; program folder: AtariRTC3), I get this error almost at the very beginning of the install:

Component transfer error:
Component: US
File group: USA
File: C:Program FilesAtari
Error: Access is denied

I should also mention that the CD Key I used is one I got on a site that is primarily for getting CD Keys to replace lost ones. I don't think this matters 'cause I had pretty much the same error with RTC 1 and that doesn't even have a CD Key.

Any advice on how to get around this or atleast any ideas on why this is happening? PLEASE HELP! Thanks in advance.