I note you are using Spybot Search & Destroy.
Try uninstalling that, (disable the TeaTimer background scanner first), and see if anything improves. We find that, with today's anti-virus scanners. some combinations of anti-spyware become toxic. I know you said this only started recently, but this would be my first test.
Is Assassin's Creed a game that needs an internet connection? if so, and if the lagging started today, try later. It could be a problem with AC's servers.
My PC games like fifa 11, nba 2k11, assassin's creed start lagging today. It only started lagging after the match started (no lagging before menu screen) . It is not my pc spec that cause the lagging because I played these games for a whole month without lagging. I also noticed that my computer is a little bit slower than usual. I think the last thing I did is my assassin's creed crash and i pressed the power off button. The first around 8 times I played it, it didn't crash but after that it always crashes after 30-40 minutes i play. So I don't know what to do and press power off button, then wait for a while then press on again. And there is some new problem occur. When I'm watching video using chrome, sometimes it lag or just got no sound when the video is playing. Is it because of the graphic card or virus? I already scan with avira, malwarebyte and spybot. There is no virus according to them.