I'm going to attempt to keep a list of the various tools that I like/use and tips that I have found useful. All are Windows XP based unless specifically stated otherwise.

Tools

MSCONFIG (Start-Run-type MSCONFIG and click OK)
This useful system tool, built into windows, will let you view system configuration files and items that initiate at startup. I regularly use it to check if various software items have decided to initiate their code at startup without my permission (typical examples are Adobe Reader and QuickTime), and I can then uncheck the boxes and stop them!

PERFMON (Start-Run-type PERFMON and click OK)
Perfmon is exceedingly useful as you can actively monitor a wide variety of system metrics, e.g. Processor activity, Specific process(application) activity,Network bytes in/out, Disk activity etc. If you run Perfmon you get a default graph of CPU activity, Paging and Disk Queue Lengths. By right clicking on the graph you can add new counters and modify the graph to your heart's content. Once you have your graphic setup as you want you can save it, and make a range of standard reporting elements. I use it to keep track on my wireless activity, which almost equates to my internet activity, so I can see when and how much I'm transferring without the need for the various 3rd party tools. In addition when if I encounter problems between applications I can use Perfmon to see what exactly is going on. My biggest success with it was in my working days when I was able to identify why PCs were running slow and it turned out to be some totally unexpected disk I/O from a virus checker. Great tool and again it's built into Windows.

Tips

Clean your Desktop
I frequently see people's desktops festooned with shortcuts. Personally, I create new toolbars which I put on the left and right of the screen, and set all toolbars to autohide and keep on top. This means I can keep a clear desktop yet have my icons readily available. When combined with multiple desktops, either with the free Microsoft Powertoys or the nView product on machines with Nvidia graphics cards, this makes for a very efficient working setup. Personally I prefer the classic start menu, and a desktop with no graphic.

Defrag your hard drive(s)
As my machine is permanently on I have automated the weekly/monthly defrag of my hard drives. Using notepad to create a simple .bat file with the simple " defrag c: /v " command, and then using scheduled tasks to run it overnight. If you don't leave your machine on you should defrag your active hard drives at least once a month, you will see improved performance.