Start by deleting files in the C:\Windows\Temporary Internet Files and C:\Temp folders.

To delete the Temporary Internet Files, open Internet Explorer
Go to View/Internet Options
In Internet Options, look for the section "Temporary Internet Files." Click on Delete Files
OR
Make sure that you are in the Temporary Internet Folder within Windows Explorer, the file and hard drive manager.
Now go to the right
pane and highlight the first file. Then scroll all the way down to the last
file. Holding down the shift key, click on the last file. All the files in
the window should now be highlighted. Place your mouse anywhere in the
highlighted field and click on the RIGHT button. A menu should pop up.
Choose Delete. All the files will be deleted.
Once that is done, empty the Recycle bin.

Delete Temporary files

To remove Temporary files, first close all programs you have running, then open the TEMP folder under C:\Windows.
All the tmp files are listed in the right pane.
Click on the first one (just once so it is highlighted.)
Scroll down to the last one. Holding down the shift key, click on the last tmp file.
All the files are now within a blue field.
Right-click anywhere on the blue field.
From the pop up menu choose Delete.
Then go to your Reycle Bin and right click on it. Choose Delete from the popup menu

When you have finished the above use ScanDisk to look for and correct file errors. If it finds errors, let it correct them automatically. When ScanDisk is done Defragment your drive.

ScanDisk and Defragment are found under Start/Programs/Accessories/System Tools.

Before running ScanDisk and Defragment, shut down your screen saver, if you have one and stop all programs from running.

This is how:
Right-clicking on any blank area on the Desktop and then choosing "Properties" from the pop-up menu. In the Properties dialog box, click on the "Screen Saver" tab. In the Screen Saver dialog box, click on the arrow on the right of the box under the Screen Saver section. Choose "None." OK your way out.

Also make sure no other programs are running. The best way to stop programs from running while the utilities run:

If you have Windows 95, move all items out of your StartUp menu into another folder and stop your Screen Saver from activating.
Reboot.
Run Defrag or ScanDisk.
Move the StartUp items back to the Startup folder.
Reboot

If you have Windows 98, Millennium Edition, XP
First, stop your Screen Saver from running
Then, go to Start/Run
In Run's "Open" Box, type "msconfig" without the quotes and press Enter key
When the System Configuration Utility Opens look for the StartUp Selection section.
Click on the line reading "Load Startup group items" to remove the check from the box.
Click on Apply.
Reboot.
Run Defrag or ScanDisk
Go back to System Configuration Utility and restore the checkmark to the "Load Startup group items" box
by clicking on the line.
Reboot.