I can second your problems with on-demand virus scanners. I tend to turn them off temporarily when doing defrags or backups, it can speed up things TREMENDOUSLY. BitDefender (work) and NOD32 (home) are notoriously bad wrt. this.
Kaspersky is pretty good if you enable it's "NTFS streams" features, because it uses those streams to keep a cached "is this file good" check, boosting speed quite a bit. Unfortunately, KAV uses rootkit-like techniques to hide the presence of those streams, so using SysInternals Rootkit Revealer you'll get an insane amount of file discrepancies - I wish KAV had chosen a single central database instead.
At work, on-demand scanning isn't disabled... but the cheery ol' people there don't really notice it