As a CS student and IT intern, these are desktop software I use the most:

Launchy - Mainly as a program launcher, sometimes as a simple calculator.

Google Drive/Dropbox - For sharing files between my work and home machines, and my laptop.

Notepad++ with MikTex- For doing my homework in LaTeX and compile it to PDF.

SumatraPDF - Lighter and faster than Adobe Reader

PuTTY/KiTTY - For logging to school server and doing my programming assignments.

WinSCP - For transferring files between local machines and servers.

Search Everything - For searching, well, everything on my computers.

Google Chrome - For, um, researching on the Internet.

CCleaner - For our staff computers maintenance

PortableApps on a USB flash drive with various software tools for troubleshooting.

And many others I can’t think of at the moment.