1. Disable any unnecessary programs/services from starting up at boot using msconfig, you can use it from the Start > Run command. You can also use a program to help you with this step called Soluto, http://www.soluto.com/
2. Do a disk cleanup of any temporary files, right click the hard drive > Properties > Disk cleanup.
3. Uninstall unneeded programs or programs that you no longer use.
4. Defragment the hard drive, even if it tells you it's not needed.