Laptop Is Sluggish To The Point Of Being Unusable?

In short, my Dell Latitude E4310 laptop is sluggish to the point of being unusable. I am also unable to keep the Windows Defender service running.

Last week I had a blue screen error (code c000021a), presumably resulting from an improperly applied Windows update. I was able to return to an earlier instance of Windows and successfully apply the patches.

Here's what I've checked / done (not necessarily in this order).My BIOS is the most recent (A11), and BIOS diagnostics come up clean. Several safe mode virus & malware scans have come up clean. Windows Start-up Repair found no problem. I've run a clean boot to no avail -- Advanced Search The power plan seems normal, and I've (temporarily) uninsalled all anti-virus & malware programs. I've run disk clean-up and defragged the hard disk System properties, Advanced, Performance, Settings is set to 'Adjust for best performance' My Windows Experience Index scores range from 6.9 (processor) to 4.1 (graphics). Running a Windows System Health Report doesn't return anything. CHKDSK (right-click c:, properties, tools, error-checking) did find and repair some HDD errors. I'd attach the CHKDSK and WININIT logs, but neither is to be found in either my or the Admininstrator event logs. I've run SFC.EXE three times, and each time the verification only gets to 10% then fails, stating the following: "Windows Resource Protection could not perform the requested operation."

The SFC log looks normal save for the following: 2012-09-07 22:30:28, Info CSI 00000023 [SR] Cannot verify component files for b10e9bd0c69db5027b503ea3749c28f3, Version = 6.1.7600.16820, pA = PROCESSOR_ARCHITECTURE_AMD64 (9), Culture neutral, VersionScope = 1 nonSxS, PublicKeyToken = {l:8 b:31bf3856ad364e35}, Type neutral, TypeName neutral, PublicKey neutral, manifest is damaged (FALSE). I cannot find a file or folder anywhwere on my system named 'b10e9bd0c69db5027b503ea3749c28f3'.