Can not install avast in infected computer

Avast Free Antivirus / Premium Security
1

My computer infected TR/Dropper.Gen virus and one other virus. All file exe system are get infect virus (like svhost.exe, explorer.exe, userinit.exe …are infected)

I tried to install Avast, Avira, KIS, Microsoft Esstensial … but can not install
Virus disable safe mode, ctr+alt+del, msconfig, regedit
I know it’s TR/Dropper.Gen because when I used USB in this infected computer, then use USB in clean other computer, Avira detected virus name: TR/Dropper.Gen
Today I will try Microsoft Windows Malicious Software Removal Tool

why all antivirus can install? What will I do?

2

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SUPERAntiSpyware Portable Scanner
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Hitman Pro 3 - Second Opinion Malware Scanner
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3

No, I used Today I will try Microsoft Windows Malicious Software Removal Tool
Removed virus
Now I can installed Microsoft Security Essentials
Microsoft Security Essentials scaned and disinfected virus in explorer.exe, useinit.exe (system files) (avira almost can not repair virus)
After all, I install Avast after computer is cleaned