Cannot Install/Update Anything Downloaded

[RTampa]

Hello all,

New poster, actually signed up because I've been trying to figure out this issue for about a month now. My mother-in-law just bought a new laptop. I've course I took care of updating and creating all back ups and restore discs. Anyways, the system has security installed and has very few software packages/applications installed (basics like QuickBooks, and the DVD package that came with the notebook). However, here is where the issue comes into play:

I get about the first 10 updates from MS installed for Vista (Home Basic) installed and then it all comes to a halt. Nothing will update after that. It downloads to about 63% and then gives me an error. There is an error code, but I can't think of it off the top of my head right now (on my own notebook at home). I can get that later. But, what I'm trying to figure out is what may cause this. In addition, every application or setup file that I download from any website is corrupted! I tried to download the latest version of the Windows Update Agent but can not install it due to it being "corrupted" after it has downloaded. The same goes with any and every fix I have found. I tried to renew the IP, flush the DNS, tried to flush all Windows updates to atleast get that up and running but nothing. I'm trying to figure out where to start. Any ideas? Advice? Any one else have an issue with everything that they download being corrupted or unable to run due to the website being an "Unknown Publisher" when you try to run an application rather than d/l it? Any websites/applications that can test the internet connection for dependability or other issues, despite still being able to retrieve websites and all other media? Thanks in advance.