Very Slow Internet NOF enabled

I have seen several discussions but have not seen a real resolution to this issue.  We have 5 computers running NOF ranging from Win XP to 7.  All computers started suffering from slow internet browsing about 1.5 months ago.  The only solution I have come up with is to turn off NOF, which is not a good solution when our kids are surfing.  We are at 2.3.0.20, but were previously at 2.3.0.19 when issue was noticed.  We also run Norton Internet Security.  Can anyone help with a real solution?  If not, I may need to dump this sofware and find another one.