We run around 30 (just ordinary donated, 3Gb / 2Cpu) PCs around here with (paritally) donated copies of NIS.
We are in general happy with it... with one BIG exception: the so-called "background" scans. They seem to be configured to run at rather high task priority level.
While they may require this during actual editing of suspicious files for malware removal, for SHA1 summing of parts of files etc., there is no need to be
running at this level during routine scans. It renders the machine totally useless for twelve hours. We find 1-7 suspicious files a week and it requires us to keep these PCs on 24x7 in order to schedule the scans during times when employees are not here. Again, it makes sense during malware removal not to be able to terminate these tasks. But disabling orderly process termination is completely unnecessary.... especially if you are opening the virus databases readonly.
What exacerbates the problem is this: for whatever reason, the scheduling profiles seem to occasionally be replaced during the course of ordinary nightly downloads and re-initialized with automatic scheduling. I just spent more than an hour trying to stop NIS from running its infernal scan (see attached). Our non-technical employees find their PCs useless frequently as a result of these Symantec configuration decisions and it really needs to be fixed. Even with a configured Manual scan, the scans seem to be starting automatically and require constant baby-sitting and rebooting to eliminate them.