Low system resources could be causing this along with quite a few other things. To look at your system resources Right-click on "My Computer" and then click on "Properties". Select the "Performance" tab. If your system resources are below 75 to 80 percent when you turn on your computer this is probably what is causing it and you should disable some items that start when the computer boots.
Two easy ways to stop programs from loading when the system boots in Win98.
1. Remove items in the Start>Programs>Startup folder.
2. Go to Start>Run and type "MSCONFIG" and click "OK". Click on the "Startup" tab and uncheck anything that you do not need running. Click "OK", you should be asked to reboot, click "Yes".
If you don't have your anti-virus program updated you may want to update it and run a full scan on your system.
There is also a program called ad-aware the checks your system for spyware, etc. that could slow your system down and freeze up. You can take a look at what the program does and they have a free download at:
How much harddrive space do you have?
Who is your ISP?
How much RAM is in the PC?