One of the easiest and most effective ways is to click start/run and type msconfig into the run box and hit the enter key.
Click on the startup tab and disable anything your don`t need. After you`re done, click apply ok and reboot your system.
After your computer has rebooted, you will see a window that says you have used msconfig to make changes etc. Click the little box that says not to run msconfig the next time you restart your system and click ok.
You should also check your system for spyware/viruses etc.
If you are using either Symantec/Norton, or McAfee, getting rid of these will most certainly help your system, as they are real resource hogs.
The free AVG antivirus programme and the free Zonealarm firewall are much better and a lot less of a drain on your systems resources.