For starters, please visit the basic computer maintenance post and see if you’ve taken care of all the steps listed there.

Assuming you’ve taken the basic steps, do the following to speed up your computer:

1) Open up the task manager and click on process to show all users.
2) Sort Descending on the CPU column and check out which process is continuously hoging CPU cycles.
3) If it’s an application that you can terminate(non-system process), right click on the task and click on “End Process”.

The steps mentioned above will temporarily bring your system back in control. Next:

Go to Start -> Run -> Type “msconfig” and press enter. In the system startup process tab, disable all the processes which are not needed to be started at system booting. Common programs include but not limited to – Messengers, Media players etc…

Use this tool  SpeedUpMyPC2009 to speed up your computer further.

These steps should give you a good start and should speed up your computer noticeably.