given that you already have a nagios installation, consider nagiosgraph or pnp4nagios.
nagiosgraph and pnp4nagios do a pretty nice job of plotting nagios performance data. nagiosgraph has a parameter-based approach to configuration, pnp4nagios has a template-based approach.
- both automatically detect new hosts/services whenever the nagios configuration changes
- both do graph zooming
- both provide graphs when you mouseover specific hosts/services
- both provide many ways to slice and dice your data
- both detect and graph the critical and warning levels you have already defined in nagios
- both can be embedded directly into the nagios frame for seamless, uncluttered navigation from current status to history and back
slicing and dicing the data are pretty important, imho. for example, you can view all services on a single host, or view all hosts with a specific service, or view arbitrary collections of graphs for arbitrary hosts and services.
installation is not trivial, but not difficult. a lot depends on how much you want to customize things. for example, nagiosgraph is 'install.pl' or 'rpm -i nagiosgraph.rpm' or 'dpkg -i nagiosgraph.deb'. pnp4nagios is './configure; make; make install'.
n2rrd can do some of these things as well, but it is not as polished and requires more work to configure.
rrdtool has quirks wrt data storage, and any system will have sampling issues. rrdtool does some data smoothing by default, but you can capture (and graph) maximums and/or minimums in addition to averages if necessary.
every rrdtool-based approach suffers from data/graph staleness since the schema in each rrd file is static and most systems use the rrd filename to identify the data. data are typically never lost when a hostname or service name changes; the rrd files still exist on disk. but some user interfaces provide ways to see 'stale' rrd files, others require manual housekeeping via command line. on many installations this is only an issue when initially configuring the system, but in dynamic environments (e.g. monitoring virtual machines whose lifetime is only a few months) it can become tedious.
one final note. there are actually two parts to trending: data collection and data display. if you go with a standalone graphing system rather than extending your existing nagios installation, then you might have to install additional components on your windows machines in order to collect the data.