Our company had a bad virus experience several years ago. Probably as a result, every pc has Eset Nod & SuperAntiSpyware installed. I think this is excessive and that Eset should allow sufficient protection by itself. Is there any good reason to have both installed on all pcs?
I'm going to go against the grain here and say that it depends.
What you don't tell us is if both of these products are running real-time scans or not? There can sometimes be incompatibility issues with running two different types of AV software, real-time. However, if ESET is running real-time protection and you have SAS running on-demand only, I see nothing wrong with this configuration.
In fact, have used the same exact configuration before. I currently use Emsisoft Anti-malware (real-time), MBAM (od) and Hitman Pro (od). I've also occasionally thrown SAS into that mix with no compatibility issues on Windows 7 Pro...
No there is no good reason, in fact, it is never wise to install multiple antivirus software on your computer, because they can break eachother.
They see that some program is accessing amlmost any file and checking the code, this is quite suspicious to the program and it will see the other AV as a spyware product or other virus.
Just as the others said, I would advise against the installation of two AV's. That said, AV should be thought of as "insurance". Something that's there if you need it, but the thing you never want to have to rely on.
Good security takes more than just an AV.
Good security is not a "set it and forget it" approach. It needs to be regularly monitored - Use ONE AV as insurance, but following a few simple steps will greatly increase your ability to avoid malware.