Scanning for nasties
 

On a PC with both admin and user accounts does one need to do a security scan on both accounts? Does AV software running in the admin account scan all the user account documents?

Depends how you've configured it.

Most AV software's default config doesn't even do the detailed scans you describe, just a quick scans of registry and whatnot.

Have a look through your configs and see how its setup.

And if you're still running Windows XP, then you've got bigger things to worry about and should be prioritising an upgrade from that first, because AV on XP is worthless now that XP is obsolete (for reasons I've explained a multitude of times before surrounding software APIs etc).

If the software was installed with administrator rights, it will act as expected whatever your rights. Most AV stuff refuses to install without admin rights. In general, it needs admin rights anyway to delete stuff during the cure.

A couple of products have had issues from time to time: I think ESET's online scan needed Admin rights but didn't ask for them, and maybe MBAM a while back.

So, almost certainly yes, everything gets scanned.
FOM.

Full scans are IO and CPU intensive, thus most modern software I've seen by default does not do automated full scans unless manually initiated.... instead it'll just scan the key system related areas (registry, C:Windows etc. etc.).

Change your config as I've hinted at, and you'll get full automated scans back.

That wasn't the question that was asked.

AV software will run with system privileges, and will therefore have permissions to scan all files on all drives that it has been configured to scan.

SD

That makes sense SD. Thanks for the comments Mixture,