I tend to like comparing working with computers to working with cars. I’m not sure why. I think it probably has to do with the fact that everyone has a vague idea of what mechanics do, but computers are often seen as a black magic.

So here’s a list of things that I’ve found handy to have in my “garage,” because you never know when you’re going to need them:

• Extra power cables. USB, power, and Ethernet, at least.
• Extra USB peripherals, especially a keyboard.
• A USB CD drive.
• A USB-to-ATA and USB-to-SATA adapter. You use it once and it’s instantly worth it. I have a bunch of old hard drives, and I can just throw this little adapter into the back of the bare drive, use the included power adapter, and I’ve got a “USB” hard drive made out of an internal drive. Don’t consider buying one without SATA support or it’ll be obsolete.
• A copy of the Ultimate Boot CD. It’s ancient (mostly DOS-based), so it sometimes has a hard time seeing a 2TB SATA disk connected off of USB or going through an SAS controller, but kind of like the USB-to-(S)ATA adapter, if you use it once you’ll sing its praises forever. It’s bailed me out repeatedly, and does everything from testing drives to checking RAM to doing CPU load-testing… Oh, and I recovered (!) a totally-destroyed boot sector after a botched OS upgrade once. I was flipping out trying to figure out if I’d managed to screw things up for good, and I ran one little tool that just fixed everything. I believe it has some nifty utilities for things like resetting Windows passwords, too, though it’s been ages since I used them and I’d be surprised if they worked on modern systems.
• A Linux live CD. I like Ubuntu, just because it’s easy and works on most everything. (Knoppix is an old favorite too.) It’s not for installing over things (although that’s cool too…); it’s for rescuing data. An Ubuntu Live CD will speak many more file formats than Windows. Boot a messed-up machine from this, and use your USB-to-(S)ATA adapter to copy files over to an external disk… And since it boots to a full OS and not just a rescue shell, you can do things let get it to use your wireless NIC so you can use Firefox to look up information while you’re working. (And an added bonus: use it to verify whether your NIC is bad or it’s just your OS install that won’t see it… Unless, of course, Ubuntu’s Live CD doesn’t support it, but it’s 3 for 3 right now.)
• A set of screwdrivers. Big and small. Mostly small.
• Some Torx screwdrivers. I held out for a long time, and eventually bought a cheap set at Radio Shack. I wish I’d done it much sooner. It turns out a lot of things use Torx screwdrivers.
• A whole, unformatted hard drive with huge capacity. Back everything up if things get scary, whether it’s because a drive is clicking or because you’re doing a major OS upgrade. It’s really worth the money to keep a 1TB+ drive that you never use. (And with the USB-to-(S)ATA adapter, you can get a cheaper internal drive, even.)

I used to keep a thumb drive with handy Windows utilities, too, but I haven’t done much with Windows lately. It had things like a bunch of SysInternals tools, CCleaner, Defraggler, and Recuva… Portable Firefox, and trial installers for anti-virus software. Revo Uninstaller. Back in the day I had Ad-Aware, too; not sure if it’s useful these days or not. Ninite is cool but not really meant for a thumb-drive. Actually, pretty much everything in Lifehacker’s How to Fix Your Relatives’ Terrible Computer is really good. Photorec is super-obscure and not easy to use for non-geeks, but it does its job amazingly well.