Possible Causes and Solutions for Login Issues and Pop-up Notifications

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Based on the given scenario, there are two most likely causes of the issues reported by the user:

1. Proxy settings are configured improperly: Since the technician was able to log in to the vendor's website before going to the office, but unable to do so when in the user's office, it's possible that the user's computer is using a proxy server to access the internet. If the proxy server settings are not configured properly, it can prevent the technician from logging in to the website. Additionally, if the proxy server is misconfigured, it could be causing strange pop-up notifications as well.

2. Rogue anti-malware program: The strange pop-up notifications reported by the user could be a sign of a rogue anti-malware program. Rogue anti-malware programs are malicious software that pretend to be legitimate antivirus or anti-malware software, but actually infect the computer with malware or spyware. They often display fake pop-up notifications to trick users into purchasing their software or providing personal information. This could explain both the strange pop-up notifications and the difficulty logging in to the vendor's website.

The other options - system files deleted, OS update, disabled services, and program file permissions changes - are less likely to be the cause of the reported issues. Deleting system files, changing file permissions, and disabling services can cause system instability, but they are unlikely to affect logging in to a specific website or cause strange pop-up notifications. An OS update could potentially cause issues, but it's less likely if the problem only occurs on the user's computer and not the technician's.