Spybot - Search & Destroy is a free spyware, adware and malware scanner for Windows.
You can use the free version of this tool as an on-demand scanner. If you think your PC is infected and your antivirus hasn't reported anything, then scan your system with Spybot - Search & Destroy and it'll give you an immediate second opinion.
The free version includes malware and rootkit scanning tools and Spybot's well-known "Immunize system" button. It also comes with tools for managing startup items, Registry backup and repair, and secure file shredding.
Download: Spybot - Search & Destroy 2.0 Free