Problem installing downloaded programs

Operating System: Windows 7 Home Premium

I recently got this Samsung R440 laptop and I've been having trouble installing downloaded programs, (ex. AOL IM, iTunes).
Itunes won't even appear when I run the installer. AOL will say "NSIS Error: Error Launching Installer".

I've tried numerous things: run as admin, cmd.exe (force install), and changed .exe into .msi. I'm wondering if this may be a problem with the registry, however I've even used Registry Cleaner with no luck.

Does anyone have a solution? Do you need to look at my cmd.exe (/ stat)? hijack this? anything else?

Thanks.

I would check for:
1. Viruses (Microsoft Security Essentials is FREE for home and small-business use)
2. Integrity of system files
SFC /SCANNOW Command - System File Checker
Option TWO, point three (3.)

There is also an interesting FREE solution by Kaspersky Labs called GetSystemInfo Parser 2.96
It troubleshoots with the report:

Installation Issues
Activation Issues
Computer has become slow
Update Issue
Web Browsing Issues
Blue Screens
Malware

You can read the review here or check out its website