How do you guys track sluggishness in networks w/o preliminary SNMP/netflow data?

[PliotronX]

If you've worked with networks, you have inevitably gotten complaints about general speed issues like logging onto domains, transferring files, browsing shares, cat videos on youtube not streaming fast enough.. anywho, I was wondering what you guys like to use to trace issues and possibly fix them without netflow statistics. I don't have any topologies or configurations, only symptoms and want to help people but at the same time don't want to spend the time of the guy who set it all up as he's got bigger fish to fry. The biggest question is wether one computer is encountering it or multiple and unfortunately these problems tend to not be localized.