I know that RJ-45 conectors have 8 pins, but 100BaseT only uses pins 1,2,3,and 6 to transfer data. If I used pins 4,5,7,8 and made an adapter for temporary use, could i use it as 2 network cables? I know it's best to run all new cable, but I don't have the time to run a second wire until a few weeks for now. (this is for my own personal use, not for a customer or anything). My question is will it work, and will there be a diminished transfer rate?

It will probably work but it will probably afect performance a bit depending on how heavy the traffic is on your network, you could use 2 pre made units like the siemon part# yt4-e2-e2 and not have to touch anything with the cable.

It would work, but maybe not at full speed. You'd probably be better off trying to use the extra wires for phone line use or something, and getting another cat-5 cable for another ethernet connection.

In cat-5 cabling, each of the 4 pairs of wires has a different twist rate, intended to reduce cross-talk. You can see this clearly if you strip back a foot or so of outer sheathing. The selection of exactly which wires are used in ethernet connections are intended to maximize speed.

I've heard of people making use of the 4 unused conductors, but it is not generally recommended.

Hope this helps some,
Radar

There is absolutely NOTHING wrong with using the extra 4 wires for another LAN, in fact it is in the CAT5 standard. What is not supported is using them for phone. Ringing current with old style indictive ringers may cause noise on your data.
In fact, if you only have a single network, there can't be any crosstalk since you are really using a bus and all the cables will have the same data on them anyway. The only reason you have hubs is to maintain the topology but everyone is still talking one at a time. Crosstalk that does exist is between the read and write lines.
The different twist rates are still adequate insurance against crosstalk, even if you do have 2 networks running.
Most of your crosstalk will happen in the terminations anyway. (NEXT Near End Cross Talk)
Be sure you maintain the twist all the way into the keystones and keep the wires as short as possible.

I have done that with no ill effect.
I have one cat5 4 pair cable going from my office setup to my A/V setup (previousely had plaun RJ45 males on each end). I needed two ethernet connections, so got 4 cat5 punchdown keystone jacks, and two surface mount 2 hole boxes. Each end was wired Orange Green to one jack, brn/blu tot he other (where the org/grn goes). At this point in time, I have the second pair used for stereo line audio.