Windows 7 - explorer.exe doesn't start up!

For some time I've been experiencing some problems with my PC's performance. Unsure what it was about I have put it aside for a time until I would find a solution. Today though, I've experienced a severe problem. After rebooting my PC, it doesn't want to start up anymore! It gets to the logon screen (which I do) and then there is only black screen and a cursor.
I managed to get around the problem, go to task manager via alt+ctrl+delete -> run process -> explorer.exe and it DOES work. But it is a temporary solution, who knows what happens next, not mentioning that I am not really keen on doing it every time I boot my PC. What should I do?
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A couple of things could be causing this, but try this for now:

Click Start-> Type in: "Folder Options"->Enter->Click on the "View" tab
Under "Hidden Files and Folders", check it. If it is already checked, un-check it.

A reasonable assumption would be an infection of some sort. Download Microsoft Security Essentials and run a scan. See what pops up, but the first step should be the instructions above.
The Windows screen is not coming because the explorer.exe is not running. Explorer.exe has to start so that you can see the windows.You can try out 2 things over here.
1. Try doing a system restore to previous date
2. Repair the Operating system using Windows 7 disk

Hope this helps
tomecktomeckAuthor Commented:
Thank you for fast reply.
Done the first.
Also the quick scan resulted in finding a troyan named TR/Dropper.Gen in file  C:\Users\x\AppData\Roaming\lsass.exe
Denying access, renaming it or deleting doesn't really help, since it comes back again soon after taking action.
After sweeping the net I've found a popular suggestion, to perform "sfc /scannow", although it was also said to be dangerous and could make the situation worse. Is it a good idea?
I've been somehow unsure whether to restore the system or not, because it might just as well strike back at me, but of course I'll follow your advice should other things fail.
sfc scannow is always a great idea to try, as it goes through and verifies that all system files are valid and intact. SFC stands for System File Checker, and is a built in utility of Windows to verify integrity of the Operating System files.

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tomecktomeckAuthor Commented:
Alright, problem seems to be fixed for now, though I have a feeling it might come back in this or other form sooner or later.
I've done the scans with few different antiviruses and their combined strength seemed to be enough to fight off the infection (Hijack.Shell in registry connected with explorer.exe was most likely the direct cause of those events). I've also performed SFC and done the checking you mentioned. If all that failed, restoring would probably be my next step, but as I said, it seems to have stabilized for now.
After doing those things I've rebooted few times to make sure, at first Windows had some objections about some files missing and overall file integrity, after second it performed some scan, and now it seems to be working (though if I were to judge it starts up considerably slower).

Thank you for your help!

Best Regards
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