Given you know so little it's probably safer, if you have time, to assume you know nothing at all and start using the helpful layer-system. Start with physically mapping every port using paper and pen then transfer that into Visio (there are loads of useful stencils at visiocafe.com). This way you'll KNOW, not just think you know, everything about how it's wired up. Then look at segments by getting into every switches/routers/firewalls etc config, again transpose this into Visio, making notes on port speed and setting (trunks, VLANs etc.), this is also a good time to change passwords/security-models and SNMP details for all your devices. Then move a layer up and map the IP world onto your VLANs and devices. At this point you'll have such a good understanding of what you have that you may not need to do much more other than some very general notes for each end-point (machine name, NICs/VLANs/IPs, function, owner etc).
I think that if you take this seriously you could turn a negative into a huge positive - it'll take some work though but it'll be worth it, personally and possibly professionally too.