A boot scan is a scan of your system, memory (and if wanted, hard drive(s)) that run automatically when your system is booted (ie: started or re-started). With Windows 95 and above (I believe) it will run after the items in the various start-up locations have been loaded. Personally, I find the boot scan to be a waste of time as it is one thing that will not allow other programs to load into memory at the same time (which makes sense).
