Avira AntiVir Free - I'd use it just for the sound it makes when it finds a malware.
Media Player Classic - Homecinema + PDVD9 Codecs - For your MKV playback needs.
7-Zip - A lite and good archiving program.
Office 2007 Professional - To write my weekly lab reports and to make my data tables and graphs.
Adobe Reader 9.3 - PDF reader is a must. You can go for Foxit if you want though.
I don't use the rest in 7, and I use the performance programs only in XP. They are good to have nevertheless.
CDBurnerXP - A simple and easy-to-use burning program.
Fences - An interface that replaces your desktop with "fences" (small named areas). You can organize your icons in these fences, and categorize them as you like.
BatteryBar - Gives detailed information about your battery.
CCleaner - As others recommend. It is better, but I prefer the disk cleanup service of Win.
Defraggler - From the same company that made CCleaner. A good disk defragmentation program, but, again, I prefer the disk defragmenter of Win.
Spybot Search & Destroy - "Immunization" is good. Although I'd say any experienced user would already know the sites he/she should not visit. The TeaTimer application is also good to track changes in the registry.
Acronis Disk Director Suite - One of the best disk management programs out there. Too bad it doesn't support 7 (yet).
RMClock - To run the HLT command in order to lock the multiplier at 12x.
SetFSB - To overclock (duh).
CPU-Z - For CPU values.
Hardware Monitor - For temperature values.
prime95 - For torture testing.
GameBooster - Shuts down some services to clear about 40MB of RAM. The improvement in games are minimal.
TuneUp Utilities 2010 - An easy-to-use program that modifies some settings for better performance. It has its own file cleaner & defragmenter, process manager, registry cleaner & defragmenter etc. It really made my XP work better.
Anything else, it is on my desktop computer. (Like Daemon Tools or uTorrent) I do most of my gaming on my desktop too.
Asus N10Jc @ 2.06 Ghz - XP / 7 Dual - 2GB 667Mhz RAM - WD Scorpio Blue 250Gb - FSB:DRAM 1:2 - CVV 1.1V