This is a list I put together for my laptop, but most of the programs will work on desktops as well.
Anti-Virus
AVG 7.5 - this baby runs in the background using very little resources. Right now it's using ~3 -4 Mb of memory on my 9300. Plenty of updates to help keep your computer virus free.
You can find it
here get the most up-to-date version. The paid version includes a lot of cool features too, though I have never used it.
Avast - Another good anti-virus program, that also provides a free version. Never used this myself, and the person who posted it never said what he liked best about it.
You can get it
here
Hardware Control
NHC - Simply put. This program can control the voltages on your CPU, has plenty of options to help save/conserve power making. Lots of user friendly options. If you want to undervolt your CPU, this is the tool. If you have done a pinmod (9300s) and think adding a few more voltages might help stable the CPU, this is your program.
*Does NOT work with Vista
You can find it
here
I8KFanGUI - This neat little program can allow you complete control over the speed of your FANs in your Dell laptop.
*Does work with Vista
You can get it
here
I9KFanGui - This program is basically the same as the one above, but it does have a different layout, and it's the one I prefer to use personally on my i9300. Never had a problem with it.
You can get it
here
ATi Tool - If you want to overclock your video card, this program should be at the top of your list. Works with nVidia as well so don't let the name fool you. Just make sure your card isn't locked out, and overclocking can be as easy as moving a lever up and down.
You can get it
here
RivaTuner - RivaTuner is a complete powerful tweaking environment, providing you everything you may need to tune NVIDIA GPU based display adapters. The widest driver-level Direct3D / OpenGL and system tuning options, flexible profiling system allowing to make custom settings on per-application basis, both driver-level and low-level hardware access modes, unique diagnostic and realtime hardware monitoring features and exclusive power user oriented tools like built-in registry editor and patch script engine make RivaTuner's feature set absolutely unmatched.
You can get it
here
Driver Cleaner Professional 1.5 - This program will completely remove your video card drivers. Perfect for upgrading video cards and drivers.
You can get it
here
Hardware Information
CPU-Z - Now this program is THE program for overclockers. It also works great with NHC. It will tell you information about your CPU, RAM, Motherboard, and other system devices.
You can get it
here
GPU-Caps Viewer - This program is similar to Ati-Tool and CPU-Z, only it's for your video card. (Note, you can not use this program to overclock the card) It will just tell you all the information you need to know about your GPU. Nifty program, and it's less than half a Mb download.
You can get it
here
Benchmarking Programs
3Dmark05 and 3DMark06 - This program will test your entire computer. It's better suited towards gamers, as it mostly tests the 3D aspect of your computer. So if you like to play games, you can use one of these as a reference to let you know what that new video card, cpu and/or ram did for your system. I will say that the video card upgrades gives you the most points in the benchmark though.
You can find them both
here
PCMark05 - This program will test your overall computer computing ability. This is for people who don't necessarily use their computer as a gaming machine, but for normal home usage. Very nice benchmarking program, especially if you do a lot of home work on it, such as photo editing, web browsing, word processing, ect...
You can get it
here
Super Pi - This program is for testing the raw processing power of your CPU by calculating Pi, the smaller the number the better the results. This is a great program if you overclock your CPU. You can see what those extra 200Mhz can give you.
You can get it
here
wPrime - This program is similar to Super Pi, only it does Square Root calculations. Again, the lower the number, the better.
You can get it
here
Spyware Removal
Ad-Aware SE Personal - This program will move some of the easy adware from your computer, its updated frequently and very easy to use. Just get the free version, unless you want some of the features of the paid versions.
You can get it
here
Advanced WindowsCare v2 - cleans up registry, spyware, etc, and repairs problems with the Performance Aspect of your computer. It works good enough I saw a difference in my PCMark score, so it's doing something right.
You can get it
here
CCleaner - CCleaner is a freeware system optimization and privacy tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history. But the best part is that it's fast (normally taking less than a second to run) and contains NO Spyware or Adware!
You can get it
here
AVG Anti-spyware Free - AVG Anti-Spyware Free is a free anti-spyware protection tool developed by GRISOFT for home use. We invite you to join the millions of satisfied customers worldwide who have downloaded the software and now enjoy the benefits of AVG Anti-Spyware Free.
You can download it
here
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If you have any other programs that may be useful, please post them and I'll update the list.