McAfee Securtity/virus scan

Question

It takes HOURS for the McAfee to scan this computer! It is up to 85,400 and some items scanned and is still going? It slows the puter way down while it is scanning, but I don't know how to reset it. I think it must be scanning EVERYTHING on the puter instead of just security risks? My other puter has a different virus program that scans everytime you sign on but only scans 900 and some items and is done in 2-3 minutes. What's wrong with this one?