Here are some possibilities to explain the freezing phenomenon primarily occurring at shutdown and other times.

Windows 98, it seems, has a flaw that sometimes prevents it from shutting down properly.
You can snag a patch for the problem at
http://www.microsoft.com/windows98/downloads/corporate.asp
When you get to that page, scroll down until you see "Windows 98 Second Edition Shutdown Supplement."
Clicking on the hyperlink will take you on to a new page. It is a really, really good idea to read the notes on that page before going on to the actual download
If the patch does not work, you will have to consider some other causes behind the shutdown problem.
Those other causes - which can also plague Windows 95 users - run to about 15.
But here are some of the more commonly seen obstacles to proper shutdown.
One of your programs is refusing to shut down properly.
One solution is to shut down all programs yourself rather than leave it to Windows to do so as it shuts itself down.
Another option is to do some troubleshooting to identify the program at fault.
If you are running Windows 95 remove all shortcuts from your StartUp folder.
If you are working with 98, uncheck all the programs listed in StartUp in the System Configuration Utility.
Go to Start/Run
When Run opens, type "msconfig" without the quotes in the Open box and click on OK.
When the System Configuration Utility opens, look down for the Startup Selection box.
In it click on the line reading "Load Startup group items" to get rid of the check mark in the box.
Click on Apply. The computer will reboot.
What you are doing is booting up without any of the programs that normally load when the computer starts.
If the computer does not freeze on you, you will know that one of the programs is causing the freeze.
You will have to go back to the System Configuration Utility.
This time, put the check mark back in the box next to Load Startup group items.
Then at the top of the System Config box look for the Startup tab and click on it.
Many, if not all, of the boxes in the window that opens should have check marks in boxes next to every line.
Click on the first line to uncheck the box, then reboot. If the computer freezes, go back, recheck the one you unchecked, reboot, uncheck the next box, etc until you find the culprit, if any.
If you do, you may have to reinstall the program responsible for the freeze or live without it.
If you are running a screen saver, consider it also to be a prime suspect.
The solution: Don't use a screen saver
You may have a corrupted driver.
Using Notepad, open your System.ini file.
Put a semi-colon, which serves as an ignore-this command to the computer, to the left of every line beginning with DEVICE= and including a reference to 386.
Save the file, reboot, shut down.
No problem this time?
Go back to the system.ini file and remove one of the semi-colons. Save, reboot, shutdown.
Do this until the PC fails to shut down normally. At that point you have identified the errant driver.
The device line for that bad driver probably refers to the piece of hardware the driver controls.
Either reinstall the driver from the disk that came with the hardware, download an updated driver from the vendor's Web site, or call tech support for help.
Your Advanced Power Management, or APM, utility has decided to take leave of its senses.
Disable it, both in your BIOS file and in Windows.
That little sound riff that accompanies the shutdown may be hitting a sour note.
Go to your Control Panel, and click on the Sounds icon to open the Property dialog box.
Scroll down in the Events box until you see Exit Windows. Change the sound settings to None.
Your TEMP folders may be bulging at the seams.
If the C:\Windows\TEMP folder gets overloaded all sorts of bizarre events can ensue. Clean it out.
While you are at it clean out your Temporary Internet Files folders as well