Maximum PC

My parents want to connect their house to the house-trailer next door where my grandmother lives. The distance is around 100 feet. I know cat5e is specc'd to run up to 100m, so distance is not a factor. Outdoor grade wireless equipment looks to be very expensive, on top of which I'd need to buy directional antennae to facilitate the signal.

I am primarily worried about lightning.

Should I run the cat5e through conduit or can I just bury the wire? If so, what kind of conduit?

Can I use regular twisted pair cabling for this?

What kind of lightning protection should I be using at either end of the wire? Is this even something I need to worry about?

I think you could do this with SOHO wireless equipment at that range (with a lower cost and a lot less work). But if you want to run cable you should put it in some conduit, I would think PVC pipe would be ok (make sure to put in and leave a pull line you'll be glad you did). You would want to dig a little deep though to help prevent accidental damage, so you should get the utilities out there to mark their stuff. The cat5e should be outside gel field cable. Need to protect against moisture (the conduit should protect against animals). Technically you should ground the cables (witch you would really need to use a patch panel to do that with). But I would think if you dig it deep and it's in pvc pipe your pretty safe from lightning. I know of lightning arrestors, but good ones are not cheap, and you really do get what you pay for with them. With the cat5e cable I've seen them, but again they are used with punch downs. I'm sure someone might know of a solution there for you. You should also check on the TIA/EIA standards for specs when pulling cat5e cable to make sure you don't over stress the cable (would also have all the info on grounding and cable types).

It would be cheaper in the long run to get a good N router and WiFi card for your grandmother's system - such as this router and this card. I have the same router, and can pick up the AP from my parents home - 300 feet away. It's fairly weak, but that's the range possible. YMMV, however...

Depending on the town/municipality codes, you might not be able to hard-wire this connection yourself. You more than likely will need a permit, and a certified electrician to approve it.

For best protection against the elements, ideally, the conduit would enter your home through the foundation or wall, then travel underground - below the frost line if possible to keep the conduit from heaving - to the trailer. Then the conduit could be brought into the trailer's wall - again to keep out the elements.