I have Norton Corporate Security running in my office and it does not stop everything. I couple that with Microsoft Security Essentials (free) and I have a fairly high success rate with catching infections. One will catch what the other won't. Sometimes both are triggered.
I have issues from time to time with slow performance. This is due to the Norton software scanning the system. If you keep the machine off for periods of time (such as a day or two), this can cause the system to feel the need to scan, resulting in a slower performance for a period of time until the AV catches up. MSSE functions pretty much the same way. When both are running their general scans, you may as well get some coffee. My workstation stays on 24/7/365, with a few off times. It is after these off times that I notice some issues with performance due to AV software.
Not every software stops everything. Just my ten cents.
