Well assuming you are using Firefox, which you should, you are lucky. Firefox actually has a simple way of doing this, both manually and automatically:
To delete cookies Firefox 2.0
1. Select "Tools"
2. Select "Options".
3. Select "Privacy".
4. In Private area click "Clear Now".
5. In "Clear Private Data" window put the check mark for "Cookies" and click "Clear Private Data Now".
6. Click OK.
To delete cookies Firefox 1.x
1. Select "Tools"
2. Select "Options".
3. Select "Privacy".
4. Open "Cookies" tab and click "Clear Cookies Now".
5. Click OK.
You can also set Firefox to automatically delete cookies every time you exit the browser, which is what I do. While it may seem annoying to keep re-logging in to everything, it greatly increases security.
As for you hard drive, assuming you are using Vista, you can either restore the factory image (
http://www.ehow.com/how_2134715_dell-windows-vista-factory-settings.html) (so it is brand new again), or you can defragment you hard drive (
http://windows.microsoft.com/en-US/windows-vista/Improve-performance-by-defragmenting-your-hard-disk). There are plenty of solutions to make your computer faster, almost all of which is free.
