Hi,
There are many factors which could slow your computer down. Lets look at a few.
1. Your hard disk space is filling up.
-This could lead to ineffeciency on your computer. If possible, clean out stuff you dont need and try to keep 15% of your disk clear.
eg, if you have a 80GB hard drive, try to keep about 19GB of space free.
Just remember not to clear system files.
2. You could be infected with spyware.
-If so, use your anti-virus software to clear it out for you.
Try http://www.avast.com/
or http://www.microsoft.com/athome/security/spyware/software/default.mspx
They are both free
3. Defrag
-Degfragmentation of your hard drive is rearranging files and memory so that it is more easier to access these files. Don't worry, its not that bad, you wont notice anything different except that it runs a tad faster after you've done it.
Assuming you use Windows XP
Go to;
My Computer --> 'Right click' on your C: drive and click properties. --> On the tabs above, click "Tools" --> click "defrag now" and follow the instructions.
Good luck