First off I agree with NETim... before you buy a new disk, make sure the one you have has plenty of available space and is defrag'ed.
Before you defrag, also check that your browser disk cache is empty. For simplicity's sake, this means every time you pull up a web page, a copy of it and the images is stored on your hard disk... needless to say, this can get rather large over time. There are buttons in Netscapeand IE that allow you to clear it out.
On Netscape, go to Edit, Preferences, then click the + next to Advanced. Click on Cache, and then push the buttons that say Clear Disk Cache (and Clear Memory Cache if you want).
On IE, go to Tools, Internet Options. Click the button that says Delete Files. Check the box that says Delete All Offline Content and then click OK.
After this, Defrag the disk. You probably will have to Defrag move than one time... . the program that comes with Windows does not do a very good job unless you do it multiple times back-to-back.
Lastly, buy more RAM. It's very cheap right now, and it will make an improvement in most anything you do. You cannot have too much RAM with Windows.
Good Luck... .