Eugene's observations

The absolute best way to clean your computer of all sorts of malware is to pay me to do it, but what if you want a quick and dirty DIY approach ?

How do you know you have been infected ?

Any of the following are apparent: 

• Performance is much slower than it was.  
• Popup windows appear without your doing anything. 
• Your browser home and/or search page is changed.  
• You can no longer log in to somewhere despite being certain you are entering the correct credentials. 
• People in your address book get messages from you that you didn't send.

First, disconnect the computer from your network to prevent the infection spreading further.  Do not power off as some attacks are triggered at boot time.  Now locate another computer and download some tools onto a newly formatted USB pen drive:

Ingredients.

• You will need a general scanner - try Malwarebytes free Anti-Malware first.  If you already have that running on your compromised computer go for SUPERAntiSpyware or AdAware instead. 
• Rootkits are a type of virus that can hide from almost all other programs.  You need a specific took for this type so try Kaspersky's TDSSkiller
• For cleaning the cruft get yourself the latest copy of Piriform's CCleaner.
• To confirm all is complete pick up HiJackThis too.  

Method.

Boot your computer into Safe mode (F8 will do this after BIOS display and before Windows starts). Run MSCONFIG to toggle off all non-microsoft services and startup programs then reboot normally.   Run each browser and disable add-ons you don't want.  From the Control Panel uninstall any programs that are not there by your design.  Delete the contents of your %TEMP% folder then run Ccleaner to delete those files you can.   Now it's time to run Malwarebytes Anti-Malware *in full mode* - this will take an hour or two depending on the speed of your computer and number of files.  

Reboot and now run TDSSkiller - this one's quick.  After that run CCleaner again but this time in Registry clean mode.  Now back to MSCONFIG to toggle on all those programs you earlier toggled off which do want to continue running.  Reboot.  Now HiJackThis can be used to point out anything that is left.  If you are reading this you are probably not sufficiently experienced to differentiate good from bad, so websites such as http://www.hijackthis.de/en can give you a good idea.

And then...

Let's assume you have been successful in the above.  

• Now you can reconnect to your network.  
• You must make sure you have a decent Internet security suite installed, updated and running perfectly.  
• Make sure Windows is updated as well as runtimes such as Java, Flash, Shockwwave etc.  
• Change any passwords that you believe could have been compromised to make them unique, long and complex.  
• Ensure that your day-to-day account does not have full administrator privileges.
• Record the date as your backups before this date may be infected, you don't want to restore from infected files. 

The above is a subset of what I would do having removed your hard disk drive and mounted it on a dedicated recovery rig.  I would  have taken an image of the disk initially, made multiple passes of the disk with different tools and confirmed that no other damage exists.  However, the above is often sufficient to get you out of a hole.  However sometimes the damage is so great that only a reinstallation of Windows will suffice.