If you've already been infected and didn't have an antivirus protecting you I would start with making sure you are clean and then setting up an anti-malware. Deep malware infections and rootkits can stop free antimalware programs from working and also evade detection.
Delete any system restore points you have, boot into safe mode with networking using F8 when the PC is starting up, download a good anti-malware like Bitdefender free or malwarebytes, update and run a full scan to make sure nothing is hiding, then when the full scan is finished restart into normal Windows and then you'll have your program set up. Then re-enable system restore.
Microsoft security essentials runs on Windows 7 and provides free active protection as well. Malwarebytes and some other antimalware programs give you free scanning but not free active protection.