Incorrect Digital Signatures Problem

I am having a problem with my parent's computer setup that I have never had on my own. Frequently, when downloading large programs from the Internet (e.g. Firefox for the PC, iTunes and QT7 through SU for the Mac), the digital signature comes up as incorrect. See attached pic.

Their setup is as follows:

SBC Yahoo! DSL --> DSL Modem --> Linksys WRT54G Wireless Router, with the following computers connected:

1. eMac 1.25GHz running 10.4.2 connected via ethernet
2. Gateway Craptop running Win XP Pro SP2 connected via 802.11g

The problem has happened on both the desktop and the laptop.

Any ideas what it could be? I am leaning towards a setting on the wireless router.

Thanks.

 

Thanks ITASOR.

I will give that a shot. I also found a firmware update on Linksys's site which I have applied to the router.

Kudos to Linksys. Their online chat was available today (on a Sunday); the wait was only 2-3 minutes and the Linksys rep was very helpful. I halfway expected them to blame anything but the router, but they didn't. They linked me to the firmware update and provided instructions for installing it using a Mac (when I wouldn't have been surprised if they insisted I do it from a Winblows PC).