Hi
Since it had Windows installed (and I assume authenticated at some point) the registered number should be correct for the computer.
You should check to see if the number you have is legit, and authenticated.
http://windows.microsoft.com/en-us/windows7/how-can-i-tell-if-windows-7-is-activated
Do you remember if you went through the authentication process when you first got the computer?
If you didn't then the computer may still be registered to a previous owner.
In the end you may have to buy a copy of Windows 7 to get a legitimate installation.
What have you done to get rid of the virus?
Have you run Malwarebytes, and SuperAntiSpyware?
How is it taking up space on your hard drive?
You could try and do a repair install of Windows 7 and see if it get's rid of the problem.
That can be done starting within Windows.
http://www.sevenforums.com/tutorials/3413-repair-install.html
The way I have done it in the past is to create a new User, and reboot to that user.
Next, open msconfig (type it in the run window), and set it to diagnostic setup and reboot to the new user account again.
This will load Windows with only the basics and no data from the previous install.
Now start the install process as described above from within that user account.
It will give you essentially a clean install of Windows with no hold overs from the earlier install.
But do make sure that your Windows COA is authentic before you do this, because it will want you to authenticate when you get done.
Mike