Windows 98SE doesn't handle memory managemet very well. If it is starting to use more virtual memory (which is simply space on your harddrive allocated to do the job of system memory when your system memory becomes full) then you may have an issue with the RAM itself. I'll list a program you can use to test memory towards the end of my post. For now, there are a few things we can try first.
1.Open up INTERNET OPTIONS (go to Control Panel and open it.)
2. Select ADVANCED and scroll down to the SECURITY area.
3. Put a check in DO NOT SAVE ENCRYPTED PAGES TO DISK and EMPTY TEMPORARY INTERNET FILES FOLDER WHEN BROWSER IS CLOSED.
4. Go to GENERAL tab and clear out COOKIES and FILES (make sure to check DELETE ALL OFFLINE CONTENT.)
5. Go into MY COMPUTER. right click on your C drive and select PROPERTIES
6. Select DISK CLEANUP
If nothing improves, run a little utility called MEMTEST86+
. Download the "Download - Pre-Compiled package for Floppy (DOS - Win)" version and put it on a Floppy. Then pop the floppy in your drive and restart. You could also download the iso and burn it to a disk if you would rather do that. Let it run through 2 or 3 cycles and see if it comes up with any errors.
If it comes up fine then we may need to check the status of your harddrive and make sure there aren't bad sectors on it which would conflict with virtual memory.