I'm fighting a rootkit virus right now, on my primary computer.
They are NASTY devils, but other people have fought them before, and there is help to be found on the Internet.
I spent several days researching the particular virus that is inhabiting my computer. One thing it does is prevent you from running or installing any software.
I found a registery repair utility for my OS that defeated this.
I am running anti-virus and anti-malware scans, even as I type this, on my infected computer. The software found a LOT of infected items.
Below are the URLs that I found very useful. You'll have to dig through them, to find the information for your problem, and you particular OS.
I would VERY HIGHLY recommend saving these links, against future need!
Perhaps put them into a MS Word files, which allows for clickable hyperlinks, and burn it onto a CD-ROM. Then you can open the links in an uninfected computer.
In order to disinfect a badly infected computer, you really need to have another computer with access to the Internet, and the ability to burn CD-ROMs available to you.
That way, you can get copies of the software you will need, ESPECIALLY the registry repair utility, onto the infected computer.
I spent a LONG time digging this information up, so save yourself the trouble, and save them for WHEN (not IF) you need it!
Some of these links are specific to my OS and specific virus, but you should be able to find information for your particular virus, and OS, at the same link(s) at different places on the website.
Spyware Warrior: Rogue/Suspect Anti-Spyware Products & Web Sites
|MG| Malwarebytes Anti-Malware 1.51.2.1300 Download
SUPERAntiSpyware.com | Remove Malware | Remove Spyware - AntiMalware, AntiSpyware, AntiAdware!
Bleeping Computer Downloads: ComboFix Download
Malwarebytes won`t install, run or update – How to fix it | My Anti Spyware
Virus Removal Support
FileHippo.com - Download Free Software
Bleeping Computer Downloads: RKill
RKill - What it does and What it Doesn't - A brief introduction to the program
Preparation Guide For Use Before Using Malware Removal Tools and Requesting Help
Remove XP Antivirus 2012, removal instructions
How to remove XP Security Tool 2010, XP Defender Pro, and Vista Security Tool 2010 (Uninstall Guide)
XP Security 2012 (Virus Removal Guide)
Good luck, if you ever need to do this.