Securing Windows 7
From CVL Wiki
Install Antivirus (Microsoft Security Essentials)
Update Flash (http://www.adobe.com/software/flash/about/)
Update Reader
Update Windows Update
In case of virus - Scan with Malwarebytes, etc http://www.microsoft.com/security/scanner/en-us/default.aspx
Run as a standard user, use the UAC (ask for admin password on install, etc)