This sounds like a simple hardware upgrade problem. You might want to check how much memory Win98 requires and then check how much memory your system has. If your system says that there is enough memory, you may need to do a clean boot that doesn't have a lot of things running in background.
Now if you can't understand some of the stuff mentioned above because you're not a PC technician, you have a couple of options.
One is to have a PC tech check the machine and determine what type of RAM you need to get the machine Win98 ready, assuming that you're RAM deficient. Note that the technician should also check what loads during the boot process, just in case the RAM problem is caused by software.
The other option is to find a bookor web site on how to upgrade your PC and do it yourself. That will probably take longer and you may make mistakes, but it improves your knowledge base.
The choice is yours.