It’s always useful to have a USB stick handy loaded with useful tools, some of my suggestions are:

Windows XP Service Pack 2: Link Here
Advanced Office Password Recovery Pro: Link Here
Adobe Acrobat Reader: Link Here
AVG Free Anti-Virus: Link Here
Checkpoint Zone Alarm Free Firewall: Link Here
Spybot Search & Destroy: Link Here
Adaware SE Personal: Link Here
Softperfect Network Scanner: Link Here for network information
SiSoft Sandra: Link Here for benchmarking and diagnostics
Tufftest Lite: Link Here for hardware testing
The Font Thing: Link Here
Transmac: Link Here for reading Mac Media on a PC
Crossfont: Link Here for converting fonts between Mac and PC
WS FTP Pro: Link Here a good solid FTP program
Notepad2: Link Here a good replacement for Notepad
Net Stumbler: Link Here for wifi analysis
WireShark (Ethereal): Link Here for detailed network analysis
Samba: Link Here for mounting Unix drives in Windows
Active@Kill Disk: Link Here for really erasing disks
puTTY: Link Here for a simple, free, terminal emulation utility.

If you have suggestions please add them using the comment facility. If I come up with any others I will add them above.