Gaming Laptop Processor Massively Underperforming

I have a Alienware M15x laptop with system specs I have until now considered to be quite good. I recently had to reinstall Windows, a bad Torrent download corrupted my computer and I didn't have a Windows Recovery Disc. Many people tell me not to care about the Windows Experience Index, but here's the rating: It used to be 6.0, 6.6, 5.5, 5.9 and 5,9. Now it's 4.1, 6.4, 5.1, 5.5 and 5.9. The one that has lowered the most is the Processor component. Now, if my computer's performance hadn't been affected, I wouldn't have bothered. However, it did. I used to be capable of running GTA 4 at maximum settings at 1980x1080 without lag, Crysis 1 and 2 at medium-high settings at that same resolution, as well as many games such as Batman: Arkham Asylum, Bioshock, Borderlands, F.E.A.R. 1 and 2 and Left 4 Dead 2 at varying quality settings. Now, even at the lowest settings I can't run any of these games without a very noticeable stutter, often making it impossible for me to play these games.

I have a Page File set for my two disk drives (I have only one Hard Drive, partitioned to separate my games from the rest of the system).I have Avast! as my antivirus, and I regularly scan my computer with Avast!, Malwarebyte's Anti-Malware and Windows Defender, but I never find anything. I also use CCleaner often to clean up the registry.What made my laptop cease to be as powerful as it was before? Was it my reinstalling of Windows? It's really getting on my nerves, seeing that I could play Crysis 2 and it was silk-smooth, but now even games like The Ball, which isn't especially resource-consuming, often stutters and lags when something's happening on-screen.