
In some case you may need to try scanning in safe mode.
AVAST ANTITRACK PREMIUM TAKES FOREVER FULL
To speed up a scan, uninstall unnecessary programs, clean out temporary files, temporarily disable any other real-time protection tools, close all open programs, perform a Quick Scan instead of a Full one and do not use the computer during the scan. You may also want to read How an Antivirus Program Works. Note: Using two or more program security scanning engines at the same time can cause each to interfere with the other, cause systems hangs, false detections, unreliable results and other unpredictable behavior.
AVAST ANTITRACK PREMIUM TAKES FOREVER UPDATE

cab, archived, compressed, packed, email, etc) that are scanned.

Disk size and used capacity (number of files that have to be scanned).What action has to be performed when malware is detected.Type of scan performed: Deep, Quick or Custom scanning.Options to scan memory, boot sectors, registry and alternate data streams (ADS).Options to scan for spyware, adware, riskware and potentially unwanted programs (PUPS).The program itself and how its scanning engine is designed to scan: using a signature database vs heuristic scanning or a combination of both.

The speed and ability to complete an anti-virus or anti-malware scan depends on a variety of factors.
