That's not a good idea, and not so much because it's hard on the system, but because the two programs can "fight" over access to a specific file. Each one is going to want exclusive access to the file it's working on at the moment, and so if they ever want to access the same file, there could be problems. Nothing catastrophic mind, just some files may not be scanned/defraged.
While on the subject, it should be noted that defragmenting is really unnecessary. It will offer minimal, if any, performance improvements and those will be fleeting in nature. Since it has no diagnostic functions whatsoever, it really is a tool that's time has past.
if I ran the defrag. and the avg virus scanner at the same time on the computer just before I go to bed is this hard on the computer or do I have to do one then the other like I have been doing? Thanks sham1313