Helping people with computers... one answer at a time.
There are many things that can lead to the error "A Cabinet file cannot be trusted". I'll review the steps I'd take to track down the problem.
"A Cabinet file cannot be trusted" - Why?
"A Cabinet file necessary for the installation cannot be trusted" is actually the full message, and it results most commonly when folks are attempting to download DirectX 9.0c for use with a new game, commonly "The Sims 2".
Wow. Forget all the business uses of computers ... just don't get in between a person and their game. Lots of questions, and no shortage of emotion to be found on this topic.
Unfortunately, in my research I've not been able to find the One True Answer. But the problem seems to be reasonably well understood, and there are a ton of things to try. Unfortunately, many of them are not simple.
The problem arises out of Microsoft's attempts to improve the security of Windows. The software is attempting to use digital signatures to verify that what you're installing is what you think it is and not some virus, trojan horse, or other malware.
When it fails, here are things to try, in the order that I would try them:
Re-download the component you're attempting to install, from a trusted source. For example go to Microsoft's site for the latest DirectX download.
I know you're tired of hearing it, but it applies here also: scan for spyware and scan for viruses. Make sure that the databases you use for each tool are as up-to-date as possible. There have been reports of this kind of problem being related to spyware or viruses.
Under Windows XP, ensure that the Cryptographic Services are enabled and running:
Ensure that you are up-to-date with Windows Service Packs. There are apparently specific fixes in the service packs that relate to the cryptography service and could result in its failure. I recommend enabling Automatic Updates as well.
Run chkdsk or scandisk on your Windows drives. Certain types of file corruption can result in this problem.
Run the System File Checker under Windows XP to detect and repair any corruption or system irregularities.
On Windows 9x (95, 98 and Me), hit Start, Run, and run the command "scanreg.exe /fix".
Review the two Microsoft Knowledgebase articles listed below. They have a long list of steps that may help resolve the issue.
Perform a Repair installation of Windows, as outlined in How to Perform a Windows XP Repair Install.
Some resources I encountered and from which some of the suggestions above were gleaned:
A comprehensive guide on fixing Cryptographic Service Errors
Microsoft Knowledgebase: Detecting Digital Signing Issues in Windows XP
Microsoft Knowledgebase: You cannot install some updates or programs
This thread on DirectX and SP2 problems out on ntcompatible.com
A thread in microsoft.public.directx.misc entitled "The cabinet file cannot be trusted (DirectX 9.0)/Service Pack2"
A thread in microsoft.public.directx.misc entitled "cabinet file cannot be trusted"
The bottom line is that yes, this is a royal pain, and no, it shouldn't have to be this way. As I said at the outset, there's apparently no One True Solution, no simple tweak, that will make this problem go away. I've prioritized the steps I'd take above so that hopefully your issue will get resolved after a few easy steps.
Take your time and try not to get frustrated (OK, try not to get too frustrated).