It is a boot-time scan, which runs when you first install avast to check that you system is clean. It can also be scheduled to happen on the next bot (one of schedule).
It is normally used when removal of a virus chant be achieved because it is protected, running or in a system folder and as such protected by windows; the boot-time scan runs before windows starts so whatever was running isn't and that interference/protection isn't present.
If the escape key doesn't work it could be that you are using a USB keyboard ?
If so it would indicate that USB devices aren't enabled in the BIOS set-up.