It would help to know more about the "weird screen flash". (Does it look similar to this?) You say "certain webpages" take forever to load. Perhaps you might share some details with us, about the types of sites/pages they are? Any other "flaky" symptoms?
With that said (or asked), have a look at what was posted at Symantec's Connect Forum. It references a SEP user, who received a BlackHole Exploit alert. However, there was no indication of it in their incident or risk logs. Sound familiar?
Technical support offered the following:
'Check the Security log on the SEP client.
The IPS should've blocked this. What the user probably saw was the notification from the SEP client that traffic from x.x.x.x was blocked (or something similar)
If that is the case and the traffic was inbound, than the IPS was doing it's job and no further action should be needed (you can do some forensics to see what sites were visited and which one may have caused the alert). As a precaution, run a full scan in safemode.' When you ran the scan with SEP afterwards, did you run it in safe mode?
See this thread, regarding "BlackHole Activity" at Norton's community forum. It may (or may not) give you the information you're looking for.
If it's any consolation, when posting news at this forum, I've found Symantec has written "more than most" about it. One example being, "The BlackHole Theory".
Try scanning with [url=
http://www.eset.com/us/online-scanner ]ESET's Online Scanner. Their FAQ and Help sections should answer any questions you might have. In order to avoid a conflict, temporarily disable SEP just prior to running the scan.
It's the best I can tell you, at this point. Short of giving you a list of additional tools to scan with.
Symantec Endpoint Protection (SEP) popped up the above message on my XP Pro machine the other day. I quickly closed the browser, and shutdown the machine. After rebooting, I checked the SEP risk & threat logs, which showed nothing, then did full scans using SEP, then Malware Bytes, and as luck would have it, the MS malicious software removal tool for April. None of the scans turned up anything. But ever since then, the machine has been acting a little flaky. There's a weird 'screen flash' during the boot process before Windows loads, and certain webpages now seem to take forever to load. How can I detect if a Blackhole toolkit has been loaded on my PC? And how can I get rid of it if it has? Thanks.