Most likely, you have a memory leak.
There are a number of memory managers that you can use to clear RAM from time to time.
Mem Turbo is one.
There are links to several memory managers in the System; Power Tools section at www.nonags.com
What you really should do, is eliminate the leak.
For Windows 98, I recommend NOT installing IE 6. I have seen it cause memory leaks and system slowdowns.
Same for DirectX9.
If you have a newer video card that supports DirectX 9, then downloading an updated driver may fix the problem.
The 'best' way to eliminate a memory leak is a fresh clean install of Windows.
Functionally, Windows 95 and 98 have a life of 6 months to 2 years before the registry and system files grow to the point where the system slows down and can have memory leaks and related problems.
If you decide to perform a clean install, here are a few steps to take:
Create a system profile. Use the free download at www.belarc.com
Print this out.
Back up ALL critical data and settings.
Download the latest drivers for ALL hardware.
Boot from a Windows startup floppy.
Format the C: drive
Insert the Windows CD and run setup.
Choose custom for the installation type and select only those Windows components that you desire.
Chas