PC Services Optimizer (formerly Vista Services Optimizer) is an open source Windows optimization solution that enables you to tweak your Windows services in an easy, automatic and safe way based on the way you use your computer.
Advanced Vista Optimizer 2008 is a complete set of utilities and tweaks to manage all areas of Windows Vista/XP. This special product will allow you to perform all those operations on your OS which were impossible using Windows interface.
Smashing Memory Optimizer is a free Windows memory optimization and process management tool! Optimize Memory to Speed Up Your PC with Smashing Memory Optimizer.
If your computer is running slow even when you have good amount of Random Access Memory, that is RAM, then use memory optimization tool to find and remove memory leaks.