When things go wrong with your Windows PC, or someone else's, you don't want to fish around or google for the proper tool. Have it right there when you need it, and you'll feel clever for it. Carry a USB drive with a variety of tools on it. And they turn out to be handy more often than you'd think.
One thing to consider is that many of the tools are bootable. Booting off a USB drive is much faster than booting off a CD, and they're writeable. But many older systems can't boot off USB media, so for those systems you may need something else to boot off of, probably a CD drive.
Windows System Repair Disc (CD/DVD only). Windows 7's Backup and Restore program includes a feature to make a system repair disc that boots into the System Recovery Options menu. The menu contains options for Startup Repair (for instance missing or damaged system files); run the System Restore utility; Recover the system from an image file; run the Windows Memory Diagnostic Tool; and run the Windows command prompt. This same menu is on the Windows installation or recovery disc (and is there for Vista as well, although Vista can't make a System Repair disc).
Antivirus Tools. Symantec's Norton Internet Security can create a bootable CD or USB that updates definitions. AVG has a free CD- and USB-based scanner. BitDefender also has both CD and USB. Avira has the free AntiVir Rescue System.
One more antimalware tool worth having around is Malwarebytes. It's not bootable, but it often finds malware that other tools won't.
Password Tools. It accidently happened that you are going to have your work done, turn on the computer, but when you input the password for the Windows login account, you just find out the password is no longer works. Passware makes a variety of outstanding password recovery tools. Many are bootable tools to reset the local administrator password.