How to Speed up your Windows Computer

Free up disk space using Disk Cleanup
Speed up drive with defragmentation using Disk Defragmenter
Check drive for physical bad sectors using Disk Defragmenter
Keep computer up to date with Windows Update http://windowsupdate.microsoft.com/
Keep Anti-Virus up to date. Run a full scan to confirm spyware and malware are removed.
Use Readyboost on computers (Vista or Windows 7) with little physical memory (USB memory stick to extend memory quantity)
Uninstall unwanted programs
Check what applications load on start up and review what is actually needed. Use Msconfig.
Review Services that are started. Research each one and confirm that are needed for your computing scenario.

Cheap Hardware upgrades to speed up your computer

Check your Hardware specifications using System Information. Review Motherboard for hardware limitations.
Load up your regular programs and note the total memory usage
Consider more memory if over 66% of physical memory is in use
Consider more drive space if less that 10% is available
Consider a graphics card upgrade if video viewing, video editing or gaming is important.
Consider dual, triple or quad processor if multiple applications are run simultaneously. This includes Anti Virus.

Consider external storage or network attached storage to extend a computers storage capabilities.
Consider multiple monitors connected to a single desktop PC instead of multiple PC's.
Use Virtual Machines to try out new operating systems.
Use Virtual Machines to assist in Server development for Physical server deployment.
Extend a physical server usefulness by using running multiple virtual machines on it. Up to 15 to 1 is possible.
Consider Cloud computing for short term scaling of server requirements.
Consider keyboard video mouse switches for multiple computer control.
Thin clients can be deployed in their 100s and be controlled from 1 blade server.