First, try to localize the "slowness" if possible.
Are all of the complainers on the same segment? or the whole LAN, or just users that cross the WAN?
Is the network slow for users just going to the Internet, or just slow for users of the internal resources?
Some common "slow network" causes (depending on who is complaining):
One or more connections are having duplex-mismatch problems. This will usually cause extreme slowness. If the affected segment feeds a server, or an Internet gateway, everyone will see it.
Many times a "slow network" is really an internal resource that is slow to respond (maybe overloaded, maybe poorly written, old drivers, a user-class machine being used as a server)
DNS issues can make a network appear slow. Something like the primary DNS is down/offline/unavailable (or duplex-mismatched) so each request is timing out on the primary and failing over to the secondary ... the delay would appear something like the first web page takes "forever" to come up. Try another DNS server as primary.
Network (LAN) overload from virus/trojan/worm activity - do some virus scanning on the machines with complaining users.
Spyware - run Ad-Aware and SpyBot Search & Destroy on the machines of complaining users
WAN overload - if you use Frame-Relay, talk to your provider and ask for utilization stats for a day, or a week. They can usually provide a utilization plot for the requested time period (request it in advance, historical data may not be available)
Bad cabling - if you made your own cables, jumpers usually, and didn't use the EIA/TIA standards ... or if the cable are not terminated well, or damaged (crushed, severe bends, twisted, knotted, piled up behind the desk), it can cause a significant negative impact on the data transmission. Try replacing the cables with some "store bought" cables.
When a user complains of slowness, substitute your laptop for their PC and see if it's still slow. IF it is, then capture the traffic with Ethereal/WireShark (it's free) and look at what the traffic is actually doing. Lots of re-transmissions? Lots of CRC errors? lots of collisions / late collisions? "Sniffers' will also give you a timeline ... you can see what the actual response time is to/from a resource to help isolate the bottleneck.
Pings & Traceroute are sometimes helpful, but depending on your firewalls & fiters, may be blocked.
Check the utilization of your switches and routers. An overloaded switch or router will cause undo latency. Check your software/firmware versions and update as necessary.
Users using streaming applications, like BitTorrent, music downloads, video downloads .... find 'em and kill them (or at least get them fired). Make sure your Network Usage Policies are enforced. If you don't have Usage policy, make one, get the upper management to sign off on it, and start enforcing it.
There's lots more, but these can get you started.
Good Luck
Scott
