If you are behind a router, it will almost certainly include a hardware firewall, so just enable the Windows firewall. AVG 8.0 is a popular choice of free av software:
http://free.avg.com/ww.download?prd=afeA spyware scanner would be a good idea. These two are very highly regarded, and free:
http://www.superantispyware.com/http://www.malwarebytes.org/mbam.phpGood luck!!!