If the processes are taking up a lot of CPU resources then killing those processes. However be careful when selecting the processes to kill. Killing system processes can make your operating system unstable, and can force your computer to reboot itself.
If your system often is running slow, take a look at what software is running when the system starts up. By removing software from the startup group (Start > Programs > Startup) the computer will have less things running, and will therefor probably run faster.
Depending on how much RAM you have installed a RAM upgrade is usually an inexpensive and easy way to increase performance.