Avast is good, and it includes anti-malware. I assume you've made sure that you have the latest updats. I don't know how computer savvy you are, so I'll give you step-by-step instructions. If you are knowledgable about these things, then you'll understand exactly what I'm saying (and have perhapse already tried it). If not, just follow the instructions.
- Click on the Start button and select Run
- In the window that pops up type msconfig and press enter
- In the window that pops up, select the Startup tab
- In the list of programs that start automatically, you will see many that you recognize, such as MS Office, Adobe Reader and others. Go over the list, and remove the checkmarks from anything you don't recognize and click on OK
- Reboot your computer into safe mode (press F8 whiile it's booting)
- Run a full system scan (or whatever Avast calls it, I've never used it myself) and remove anything it finds.
- Reboot your computer normally
You may find that some programs don't start automatically any more, or they take longer to load. In that case, go back to msconfig and find the correct check box to fix it. If you find you still have malware on your computer, I suggest you download the free trial version of Bitware Defender and let it scan your computer. There's nothing wrong with Avast, but sometimes one anti-virus will find something that another one will miss.