To speed up your computer and free up space and memory for free:
Go to the Start Menu> Run
Type in "appwiz.cpl"
Uninstall whatever you don't need.
Next, go to Start> Run and type in "msconfig".
Go to the Startup Tab and uncheck everything you don't want to startup. If you want, you can uncheck everything, and your anti-virus will still restart itself when you log on.
Now go to My Computer> Hard Drive> Documents and Settings> [USER NAME]> Local Settings> and delete everything inside the Temp Folder. (This is a hidden folder so you will need to have "show hidden files and folders" checked in Folder Options) All the files in this folder are just temporary files that your Internet stored for certain websites.
Next, you should install CCleaner; it will get rid of alot of temporary files on your computer. It also has a registry repair system, which is easily the best free one available on the Internet:
http://www.ccleaner.com/download
Another option is to try defragmenting your computer.
Go to the Start Menu and go to All Programs
Go to Accessories and System Tools.
Choose Disk Defragmenter and when it loads click defragment.
Next, go to Device Manger (Start> RightClick My Computer> Properties> Hardware Tab> Device Manager) and expand all listings. Now rightclick and update all drivers.
Another thing I would say is to compress your files if you already haven't.
Open My Computer and right click on your hard drive
Check the Compress Drive button and let it do its thing.
Finally, make sure your Hard Drive doesn't have any bad sectors. Go to My Computer> Right Click Hard Drive> Properties> Tools Tab> Check Now> Check both boxes and Start. It will ask you to reboot, so reboot. This will clear out any bad sectors on your Hard Drive.
These are short-term solutions however. The easiest way is to just upgrade the RAM on your system, but at least the ways above are free. And honestly, as long as you have at least 15% free space on you hard drive, it won't slow your computer down, just the programs taking the space will. If you are interested in making your computer faster, I wouldn't recommend installing Windows security updates, as these will pill up on your computer. They make the computer safer and more stable, but not faster.
Good Luck =]