There are several free antivirus and spyware tools

I use the Free version of PCTools Spyware Doctor with Ant-virus from
Google. In conjunction with Threatfire from PCTools. They work well
together. For older systems with low resources Clamwin works well
with Threatfire.

During installation of the Free PCTools Spyware with AV
you will be prompted to install the google toolbar - I usually say no.
You should enable community options with threatfire install

PC Tools
http://www.pctools.com/

Threatfire
http://www.threatfire.com/

Threatfire Tutorial
http://www.threatfire.com/tutorial/Default.aspx