I see a process in task manager called "System", and I couldn't find any kind of relevant explanation about it on the net. Task manager shows its description to be "NT Kernel & System". Could someone explain what is this process and how does it relate to the other core processes of windows 7? Thanks!
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From Windows Internals 6th ed. Part 1. page 69
System Process and System Threads
The System process (process ID 4) is the home for a special kind of thread that runs only in kernel mode a kernel-mode system thread. System threads have all the attributes and contexts of regular user-mode threads (such as a hardware context, priority, and so on) but are different in that they run only in kernel-mode executing code loaded in system space, whether that is in Ntoskrnl.exe or in any other loaded device driver. In addition, system threads don't have a user process address space and hence must allocate any dynamic storage from operating system memory heaps, such as a paged or nonpaged pool.
If the process is called System with PID of 4 then it is the "Web Deployment Agent Service"
If it's not with pid 4, occupying port 80 then it's just a part of the kernel handling the drivers that interact with the system hardware. Killing it will make your computer BSOD as well, as you experienced. I don't have any official microsoft reference right now, maybe someone could dig it up.
If the process is called System.exe:
Most likely it's a trojan backdoor + keylogger. I've had it in the past as well.
Here's a description from Kaspersky that should fit - hopefully. If it's not a variant of these viruses, still check the registry keys. I have added the description of PSW-Delf just so you can have a general guideline on how to remove it and handle the registry as well.
Before performing any manual checks I recommend downloading / updating antivirus + spyware/malware programs and running a full scan (boot scan, memory scan, file+registry scans).