Over the past 18 hours or so the Nexus sites have been sporadically serving a trojan file through the ads on the site. Thanks to the logs of a few astute members on the forums I was able to identify the source of the trojan attack and have hopefully now nullified it. The Nexus sites should, touch wood, no longer be serving these compromised ads any more.
Such attacks have become natural on the internet and the fix is often reactive to an attack rather than preemptive. Indeed the patch to fix this issue was only released after several big name players in the internet community had been infected by the attack. It's simply wicked people, normally from Eastern Europe and Asia, preying on individuals who aren't up-to-date with their internet security.
Please insure you take the necessary steps to properly protect yourself from such attacks. Get a good reactive anti-virus (I used AVG and have had no trace of infection despite being warned about the threat) and firewall and switch any settings you might not need to an "ask me" state.
My apologies for the breach of security on these sites.Update: Google is reporting the site as malicious right now. This is a report from before the fix and despite the problem being fixed there is some lag on getting delisted from Googles filters. Their webmaster tools report the site is completely clear of Malware so this should just be a waiting game.Update #2: It's now Sunday morning here in the UK and Google are still yet to unblock TESNexus from their blacklist. Fallout 3 Nexus was on their blacklist for around 8 hours and was removed early yesterday. The irony is Fallout 3 Nexus and TESNexus use the same adserver and ergo had the same issues, so if Fallout 3 Nexus is clean, so is TESNexus. Their Webmaster Tools system continues to tell me the site has been inspected and confirmed clear of issues. Please be patient while we wait for Google to pull their thumbs out.
In the mean-time if you are confident in your system's security then HugePinball has written out how to remove the false malware warning when browsing TESNexus.