How and where is this site hosted? For example, are you paying for a VPS and you took the time to install the forum software yourself? What third party plugins are installed on the forum? Could any of them be out of date? What third party scripts are attempted to be loaded? Are any on non-reputable CDNs?
This is not information that we would typically release to the public. I do understand that you're looking for a point of entry for malware on the site. However, I'm with Kabepo. If this was coming directly from our website, then everyone would be experiencing it and we'd have a ton more reports about it. Not only that, but our hosting provider would also likely catch on to the activity. The only reason this wouldn't be widespread and still coming from our website is if it was a very targeted attack (region, browser app, search engine, etc). Else, the most likely explanation is as Kabepo says; a local issue on the local system.
My first recommendation here is to run SuperAntiSpyware. I've used it for years while doing freelance computer repair. Download the portable version, run your system in Safe Mod without Networking, then run the scan. Clean up anything it finds. I personally do not care for MalwareBtyes.
I personally use MS Security Essentials and just run SuperAntiSpyware once every couple months. Good practices is where it's at. I never have any issues because the common tracking cookies. I once used Avast, but found it unnecessary.