CAT5 is Category 5 cable. The cable contains a total of eight wires, as four twisted pairs. Each wire is skinny, 24 gauge wire. It is typically used to connect computers and computer components such as DSL modems. Thus it is designed to carry high frequency, small current signals.

It can be used as speaker cable, but it is not designed to carry this much current. However if you have have multiple runs of CAT5 and can use one cable for each speaker, using two pairs for positive and two pairs for negative for connecting each speaker should be OK for a setup with modest power.

CAT5 could serve just fine when used to route line level signals to powered speakers (e.g. AudioEngine). You didn't mention how this cable is routed in the walls. For normal computer application, each CAT5 cable would be routed "home run" to a central wiring closet for connection to a switch/router. It's hard to see an application for four computers in the family room though. Calling an electrician sounds like a good idea.

Just to add to that, Category 3, 4, 5 and 6 cables look pretty much the same. The difference among the categories is the shielding. The higher the category, the better the shield. Category 3 cables were used for 10mb Ethernet. When 100mb Ethernet was developed, Cat 5 cable was introduced to provide better shielding to maintain 100mb throughput over long cable runs. If I'm not mistaken, Cat 6 is used for Gigabit Ethernet cabling.

If you've heard or read about someone using Cat5 wires for speaker cables, they're probably doing something like this:

http://www.venhaus1.com/diycatfivecables.html