Often, once infected, it is too late to install Antivirus software. The installation or the updating will fail.
I would recommend following the Preparation Guide to posting a High Jack This log. You may not need to actually post a log, however. If you follow the instuctions in the Preparation Guide faithfully then your computer may be cleaned. However, rarely is a computer infected with only one thing, and it is possible that not everything nasty will get removed using this method.
If after completing all of the steps in the Preparation Guide you continue to have issues, post a HJT log as instructed in the Preparation Guide.