Since AZiNEC seems to have covered the anti-virus software part of it but I will explain manual removal in case that doesn't work.
A) Delete ALL executable files you have downloaded.
B) Get Process Explorer (See link in sources) and run it.
C) Figure out which process is the Virus (can use google to verify each one)
D) It will tell you where it is running from (won't be the original file), write the files location down.
E) Kill the process
F) Start --> Run --> type "regedit"
G) Ctrl+F and search for "runonce" with the search set to keys
H) Just above RunOnce should be Run click on it, this will list all things that run when your computer starts up and what file it runs
I) Check the entries against the what you wrote down in step D, delete the one that matches
J) Then find the file from step D and delete it.
Once you have removed the virus you're going to need to change your password, these viruses work by getting your password then feed your account into a server to send emails thus until you change your password they still have the correct credentials to log in and send emails.