I received a computer with the same error today. I am not sure if it is the exact problem, but it wouldn't hurt to try the same fix I used. I have repaired this computer. Is your control panel missing? This computer was missing the control panel and would give me the same error message, "The system administrator has set policies to prevent this installation", anytime I tried to download anything.
Because I couldn't download anything I used my computer and put the following on a flash drive.
I transfered these to the desktop and then went into Safemode with networking.
First I ran Superantispyware and it found a rootkit.reactor-variant
then I ran Combofix and rebooted to finish in Windows
Then I went back into Safemode with networking and ran SDFix
After I rebooted the control panel returned
Then I ran AVG and Spybot
unchecked the system restore, so I didn't back up any problems and then went and set a new system restore.
Everythings seems to be running great, but I will be running more test to be sure.
The most important fix was the SDfix, so you may want to try it first and see if you get somewhere with that. Here is a link with directions for SDfix:http://forums.spybot.info/showthread.php?t=8538&page=2