Avast is extremely non-intrusive. You can also schedule boot-time scans by which it will quarantine/remove viruses before windows even loads. This way, if you suspect you have an issue, you can run the boot-time scan and it can detect and quarantine/remove the virus before it starts running in the Windows environment. It will also scan your system restore files.
I have not had to re-format a system since I've had this running on all my PCs (6) the last 4 years.