~PR-Imagery is right. Open task manager Cntl+Alt+Del > file > new task run > cmd.exe and type the command: sfc /scannow This will scan for corrupt or missing system files and should fix it. (When the process is finished, reboot the computer).
Ok, so here's what I have done in more advanced terms: I opened the Windows start button changer and when I was loading the file, instead of loading a bitmap file which program wanted, I selected a PNG file. So in the resources section of my explorer.exe is now a PNG file instead of the ordinary uncompressed BMP. Now that's the first part. The program creates a backup of the original explorer.exe before patching it. But the program messed up something probably because I loaded a PNG instead of BMP and made two backups of the explorer like this: So, in the first step, it made a backup of my original explorer.exe and it patched the current one, but in the second step, it took that patched file, and made a backup of it by overwriting the original one. The patched explorer.exe throws an exception (crashes) every time I run it with an exception number 0xc00007b and by Googling, I couldn't find anything useful about it. Now I'm gonna load the damaged explorer.exe in IDA debugger and try to see exactly what's happening in there. And I'll try digging in with the ResHacker to see is there really a PNG. Thanks everyone for the tips.
PS. My system restore is off so the SFC isn't working. Also, I don't want to do a full reinstall and waste a lot of time, that's why I'm asking only for a single file that I can replace and it'll work immediately.