Stuff breaks. Hardware, software, users-everything eventually needs to be fixed. Take a look at any tech guy's (I use that term with no gender bias-girls can be "guys" too!) usb stick and you'll get a glimpse into how they fix what's broken. Remember: these programs are powerful, and with great power comes the ability to screw up your computer in less than 10 seconds.
ComboFix
ComboFix has already saved me countless hours of searching a computer for odd activity. It picks up on all sorts of random stuff that antivirus programs like Symantec miss. Their site has a great walkthrough where you can also download the software. They update it regularly, so while it doesn't hurt to keep a copy on a flash drive, you'll want to download the latest version when you have the ability. You may also have to set your region settings back after using it to get your clock to display normally.
SysInternals
The suite of SysInternals utilities is the brainchild of Mark Russinovich and Bryce Cogswell, who have both joined Microsoft when their company was bought in 2006. Microsoft continues to host their software, giving implicit approval of their utilities. They've got utilities for everything. My favorites?
Process Explorer - tells you everything you'd ever want to know about what's running on your computer. Task Manager on methamphetamines.
Rootkit Revealer - catches rootkits by scanning for differences between raw disk info and what's reported through Windows APIs.
Autoruns - like Nirsoft's strun utility, shows all programs that start with the computer. A little too much info for end-users, but great for troubleshooting.
PsTools - a bunch of command line tools for working on a computer remotely.
Nirsoft
Whew-where to begin! Password recovery tools, network monitoring tools (CurrPorts!), and a ton of other tools. Definitely big on the password recovery stuff, to the point that Symantec routinely quarantines some of the programs as being capable of stealing passwords! Verboden equals must have.
What am I missing from my list? What do you have in your toolbelt?