Lightning hitting your cat5 cable is very unlikely.
You really won't have a cable that is safe against lightning. STP (shielded) doesn't protect against lightning either, the shielding actually protects it from getting interfence from running the line with electrical wiring through a building. If lightning is going to strike, its going to strike and do its damage. More than likely if your garage is struck, the lightning will never have a chance to pass into your cabling because I'm assuming your garage is probably wooden and already grounded. If you bury the cable lightning shouldn't be an issue at all.
I was talking to another guy I work with about this posting. He's running 110 feet of UTP out of his apartment window, down the wall, along the sidewalk and into his garage. He's leaving the wire exposed but used an edge trimmer to get a few inches down along the sidewalk. He moved out into a house about a month ago and the line was still in great condition after being exposed for 2 years.
If you bury the piping/cable down at least 2 feet you should never have to worry about cutting it unless a waterline/gasline/etc breaks, those tend to be buried deeper. Should you have an underground sprinkler system installed, they shouldn't need to go deeper than 12-14 inches to install it.
As far as the use of STP, you can read up on in at this link:
http://www.informit.com/isapi/product_id~{967563CD-A039-47B7-85CB-C5A8485C4644}/content/index.asp
Just over halfway down it will state the uses of STP, mainly used in Europe and what it does.
STP is out of the question, not needed.
As far as certifying wiring, I'll explain this.
If I run a cable 200 feet from point A to point B, I would need to use a cable tester which reports what quality of MHz I get from the cable. You can buy cable like Cat5 which is 100MHz, cat5e which is like 200Mhz, Cat6 goes up to like 400MHz, off the top of my head I'm not sure since its changed so much. Anyhow, if I have cable that can run up to 400MHz, I put the cable checker on both ends, which sends a signal and tells me the MHz I've obtained. You can get a higher MHz by avoiding electrical grids, lighting, etc. If the cable comes back saying I have 370MHz, I would more than likely certify it at 300MHz or 350MHz, meaning that the cable is capable of consistently obtaining that speed and should someone come in to check the cabling, it wouldn't come across as a violation. Does it need to be certified? Only in businesses mainly, at home, I wouldn't worry about it.
Back to the UTP in the PVC piping. PVC piping is used in a lot of plumbing now-a-days, and its safe. I would suggest running the PVC piping because you may want to run additional cabling later or even a coax cable for your TV out that way. I do recommend burying it, it shouldn't be that bad digging 6 inches down, but if you want to go deeper, rent equipment or have a lot of time on your hands.
PVC is probably cheaper than the metal piping, but you wouldn't have to worry about that pipe cracking to breaking should a dump truck need to drive across to dump dirt or stones in your backyard, or whatever
.
Hope this helps, that website should let you know about STP (and how its mainly used in Europe anyhow).
UTP should be more than enough for what you want, the PVC or piping is recommended, but optional. You can run the line and bury it later or install the piping later. It comes down to how much you want to spend and future expansion into the garage. Dig a ditch once with the piping, 6 months later run a line through the pipes, you're in the clear. Don't laying the piping you might find yourself down the road digging another ditch to lay the line
Good luck!
Riser