The site may have been spoofed or otherwise been injected into your PC.

It is possible, with fairly simple tricks, to make it look like you’re visiting one site, but in reality you’re on a fake site.

I would normally trust adobe to NOT include such trojans in their software, however it may also be that adobe’s auto update feature triggers a false positive in the NOD32 AV program. But then again, I would also trust NOD32 to give me the right information.

My best advice would be to scan your computer for malware and virus, of course using your NOD32, but also use a few free services and an additional malware scanner to make sure nothing is wrong. If you require an update to flashplayer, I would then go directly to the website of adobe instead of using any links from other pages, that may have been falsified.

Should your NOD32 still claim a Trojan Horse, then for the sake of your own sanity, I would try to contact the NOD32 site to find out what’s going on. They should be able to explain the problem much better, and if their NOD32 doesn’t detect a possible malware, they’d want to know in any case. Falso positive or unidentified malware infection.

List of tools you may find useful:
http://badwarebusters.org/main/itemview/1659#itemblock-3035

Edit: link to tools.